As filed with the Securities and Exchange Commission on July 21,August 8, 2023

Registration No. 333-            

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FormFORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

CASA SYSTEMS, INC.

(Exact name of registrant as specified in its charter)

100 Old River Road

Andover, Massachusetts 01810

(978) 688-6706

(Address, including zip code,Including Zip Code, and telephone number, including area code,Telephone Number, Including Area Code, of registrant’s principal executive offices)Registrant’s Principal Executive Offices)

Edward Durkin

Interim Chief Executive Officer and Chief Financial Officer

100 Old River Road

Andover, Massachusetts 01810

(978) 688-6706

(Name, Address, including zip code,Including Zip Code, and telephone number, including area code,Telephone Number, Including Area Code, of agentAgent for service)Service)

CopiesCopy to:

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC:Approximate date of commencement of proposed sale to the public: From time to time after the effective date of this registration statement.statement becomes effective.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.  ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, as amended, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.  ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a registration statement pursuant to General Instruction I.D.I.D or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  ☐

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D.I.D filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,”company” and “emerging growth company” in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE SECURITIES ACT OFThe Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, OR UNTIL THIS REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A)as amended, or until the Registration Statement shall become effective on such date as the Commission acting pursuant to said Section 8(a), MAY DETERMINE.may determine.

EXPLANATORY NOTE

This registration statement contains two prospectuses:

The base prospectus immediately follows this explanatory note. The specific terms of any securities to be offered by us pursuant to the base prospectus other than the shares under the Sale Agreement will be specified in a prospectus supplement to the base prospectus. The specific terms of the securities to be issued and sold under the Sale Agreement are specified in the sale agreement prospectus supplement that immediately follows the base prospectus. The $10,000,000 of Common Stock that may be offered, issued and sold under the Sale Agreement prospectus supplement is included in the $100,000,000 of securities that may be offered, issued and sold by us under the base prospectus. Upon termination of the Sale Agreement, any portion of the $10,000,000 included in the sale agreement prospectus supplement that is not sold pursuant to the Sale Agreement will be available for sale in other offerings pursuant to the base prospectus and a corresponding prospectus supplement, and if no shares are sold under the sale agreement prospectus supplement, the full $100,000,000 of securities may be sold in other offerings by us pursuant to the base prospectus and a corresponding prospectus supplement.

The information in this prospectus is not complete and may be changed. The selling stockholders named in this prospectusWe may not sell these securities pursuant to this prospectus until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and iswe are not soliciting an offeroffers to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion, Dated July 21,completion, dated August 8, 2023

PROSPECTUS

$100,000,000

CASA SYSTEMS, INC.

Up to 11,907,973 Shares of Debt Securities

Common Stock

Offered by the Selling StockholdersPreferred Stock

Units

Warrants

This prospectus relates to the potential resaleWe may offer and sell securities from time to time of some or all of the 11,907,973 shares of common stock, par value $0.001 per share, of Casa Systems, Inc., or common stock, that have been issued or that are issuable upon exercise of the warrants held by the selling stockholders identified in this prospectus. The warrants were issued pursuant to the Warrant Agreement, dated as of June 15, 2023, by and between the Company and American Stock Transfer & Trust Company, LLC, as warrant agent, which is referred to herein as the Warrant Agreement.

The registration of the shares of common stock covered by this prospectus does not mean that the selling stockholders will offer or sell all or any of the shares. The shares of common stock offered hereby by the selling stockholders, or their pledgees, donees, transferees or other successors in interest, may be sold from time to time directly or indirectly through one or more underwriters, broker-dealers or agents, and in one or more publicofferings of up to $100,000,000 in aggregate offering price. This prospectus describes the general terms of these securities and the general manner in which these securities will be offered. We will provide the specific terms of these securities in supplements to this prospectus. The prospectus supplements will also describe the specific manner in which these securities will be offered and may also supplement, update or private transactions. The shares of common stockamend information contained in this document. You should read this prospectus and any applicable prospectus supplement before you invest.

We may be soldoffer these securities in one or more transactionsamounts, at fixed prices at prevailing market prices at the time of the sale, at varying pricesand on terms determined at the time of sale or at negotiated prices. These salesoffering. The securities may be effected in transactions, which may involve crossessold directly to you, through agents, or block transactions. If the shares of common stock are sold through underwriters broker-dealersand dealers. If agents, underwriters or agents,dealers are used to sell the selling stockholders will be responsible for underwriting discounts or commissions. The timing and amount of any sale is within the sole discretion of the selling stockholders, subject to certain restrictions. See the section entitled “Plan of Distribution” for more information.

We will not receive any proceeds from any sale of common stock by the selling stockholders. However,securities, we will receive proceeds from any cash exercise of the warrants, which have an exercise price of $0.01 per share of common stock. The warrants provide for the selling stockholders to exercise the warrants onname them and describe their compensation in a “cashless basis” that, if elected by the selling stockholders, will reduce the number of shares of common stock issued upon exercise of the warrants in lieu of cash payment. See the section entitled “Use of Proceeds.”prospectus supplement.

Our common stockCommon Stock is listed on the Nasdaq Global Select Market under the symbol “CASA.” On July 19, 2023, the closing price for our common stock, as reported on the Nasdaq Global Select Market, was $1.19 per share.

Investing in our common stockthese securities involves a high degree of risk. Before making an investment decision, please read the information under the headingcertain risks. See “Risk Factors” beginning on page 7 of thisincluded in any accompanying prospectus supplement and in the documents incorporated by reference intoin this prospectus.prospectus for a discussion of the factors you should carefully consider before deciding to purchase these securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is                 , 2023.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the U.S. Securities and Exchange Commission, or the SEC, utilizing a “shelf” registration process. Under this shelf registration process, the selling stockholderswe may from time to time sell any combination of the shares of common stocksecurities described in this prospectus in one or more offerings.offerings for an aggregate initial offering price of up to $100,000,000.

NeitherThis prospectus provides you with a general description of the securities we normay offer. Each time we sell securities, we will provide one or more prospectus supplements that will contain specific information about the selling stockholders, have authorized anyone to giveterms of the offering. The prospectus supplement may also add, update or change information contained in this prospectus. This prospectus, together with any accompanying prospectus supplement, contains important information or to make any representation other than thoseyou should know before investing in our securities, including important information about us and the securities being offered. You should read both this prospectus and the accompanying prospectus supplement together with the additional information described under the heading “Where You Can Find More Information” beginning on page 2 of this prospectus.

You should rely only on the information contained in or incorporated by reference in this prospectus. You mustprospectus, any accompanying prospectus supplement or in any related free writing prospectus filed by us with the SEC. We have not authorized anyone to provide you with different or additional information. If anyone provides you with different, additional or inconsistent information, you should not rely uponon it. We do not take responsibility for, and can provide no assurance as to the reliability of, any other information or representation not contained or incorporated by reference in this prospectus. The selling stockholders are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where it is lawful to do so.that others may give you. This prospectus doesand any accompanying prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any sharessecurities other than the registered shares to which it relates, nor doessecurities described in this prospectus constituteor such accompanying prospectus supplement or an offer to sell or the solicitation of an offer to buy sharessuch securities in any jurisdiction to any person to whom it is unlawful to makecircumstances in which such offer or solicitation in such jurisdiction.is unlawful. You should not assume that the information containedappearing in this prospectus, is accurate on any date subsequent toprospectus supplement, the date set forth on the front of the document or that any information we havedocuments incorporated by reference and any related free writing prospectus is correct on any date subsequent to the dateaccurate only as of their respective dates, regardless of the document incorporated by reference, even thoughtime of delivery of this prospectus, is deliveredany applicable prospectus supplement or shares are sold on a later date.any sale of securities. Our business, financial condition, results of operations and prospects may have changed materially since those dates. This prospectus incorporates by reference market data and industry statistics and forecasts that are based on independent industry publications and other publicly available information. Although we believe these sources are reliable, we do not guarantee the accuracy or completeness of this information and we have not independently verified this information. In addition, the market and industry data and forecasts that may be included or incorporated by reference in this prospectus may involve estimates, assumptions and other risks and uncertainties and are subject to change based on various factors, including those discussed under the heading “Risk Factors” contained in this prospectus, and under similar headings in other documents that are incorporated by reference into this prospectus. Accordingly, investors should not place undue reliance on this information.

A prospectus supplement may add to, update or change the information contained in this prospectus. You should read both this prospectus and any applicable prospectus supplement together with additional information described below under the heading “Where You Can Find More Information; Incorporation By Reference.”

We use various trademarks and trade names in our business, including without limitation our corporate name and logo. All other trademarks or trade names referred to in this prospectus are the property of their respective owners. Solely for convenience, the trademarks and trade names in this prospectus may be referred to without the ^® and ^™ symbols, but such references should not be construed as any indicator that their respective owners will not assert, to the fullest extent under applicable law, their rights thereto.

Unless the context otherwise indicates, references in this prospectus to “Company,” “we,” “our” and “us” refer, collectively, to Casa Systems, Inc., a Delaware corporation, and, where appropriate, our subsidiaries; and the term “securities” refers to the shares of our common stock offered by this prospectus.subsidiaries.

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CAUTIONARY STATEMENT REGARDING WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at www.sec.gov. Copies of certain information filed by us with the SEC are also available on our website at www.casa-systems.com. Our website is not a part of this prospectus and is not incorporated by reference in this prospectus.

This prospectus is part of a registration statement on Form S-3 we filed with the SEC. This prospectus omits some information contained in the registration statement in accordance with SEC rules and regulations. You should review the information and exhibits in the registration statement for further information about us and our consolidated subsidiaries and the securities we are offering. Statements in this prospectus concerning any document we filed as an exhibit to the registration statement or that we otherwise filed with the SEC are not intended to be comprehensive and are qualified by reference to these filings. You should review the complete document to evaluate these statements.

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INCORPORATION BY REFERENCE

The SEC allows us to incorporate by reference much of the information we file with the SEC, which means that we can disclose important information to you by referring you to those publicly available documents. The information that we incorporate by reference in this prospectus is considered to be part of this prospectus. Because we are incorporating by reference future filings with the SEC, this prospectus is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus or in any document previously incorporated by reference have been modified or superseded. This prospectus incorporates by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act (in each case, other than those documents or the portions of those documents not deemed to be filed) between the date of the initial registration statement and the effectiveness of the registration statement and following the effectiveness of the registration statement until the offering of the securities under the registration statement is terminated or completed:

You may request a copy of these filings, at no cost, by writing or telephoning us at the following address or telephone number:

100 Old River Road

Andover, Massachusetts 01810

(978) 688-6706

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RISK FACTORS

Investing in our securities involves significant risks. Please see the risk factors under the heading “Risk Factors” in our most recent Annual Report on Form 10-K on file with the SEC, as revised or supplemented by Quarterly Reports on Form 10-Q subsequently filed with the SEC, each of which is incorporated by reference in this prospectus, along with any risk factors identified in a prospectus supplement. Before making an investment decision, you should carefully consider these risks as well as other information we include or incorporate by reference in this prospectus and any prospectus supplement. The risks and uncertainties we have described are not the only ones facing our company. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business operations. The occurrence of any of these risks might cause you to lose all or part of your investment in the offered securities. Actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks referenced below and described in the documents incorporated herein by reference. The discussion of risks includes or refers to forward-looking statements. You should read the explanation of the qualifications and limitations on such forward-looking statements discussed elsewhere in this prospectus.

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FORWARD-LOOKING STATEMENTS

This prospectus and the documents and information incorporated by reference in this prospectus include “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. All statements other than statements of historical fact contained in this prospectus, including statements regarding our future results of operations and financial position, business strategy and plans and objectives of management for future operations, are forward-looking statements. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements.

In some cases, you can identify forward-looking statements by terms such as “may,” “will,“might,” “should,” “expects,” “intends,” “plans,” “anticipates,” “would,” “could,” “intends,” “target,” “projects,” “seeks,” “contemplates,” “believes,” “estimates,” “endeavor,” “predicts,” “potential” or “continue” or the negative of these terms or other similar expressions. The forward-looking statements in this prospectus are only predictions. We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our business, financial condition and results of operations. Accordingly, these statements involve estimates, assumptions, risks and uncertainties which could cause actual results to differ materially from those expressed in them. These forward-looking statements speak only as of the date of this prospectus and are subject to a number of risks, uncertainties and assumptions described in the “Risk Factors” section and elsewhere in this prospectus and such forward-looking statements are qualified in their entirety by reference to the factors discussed throughout this prospectus, and in particular those factors referenced in the section “Risk Factors.“Risk Factors.” Because forward-looking statements are inherently subject to risks and uncertainties, some of which cannot be predicted or quantified, you should not rely on these forward-looking statements as predictions of future events. The events and circumstances reflected in our forward-looking statements may not be achieved or occur and actual results could differ materially from those projected in the forward-looking statements.

This prospectus contains forward-looking statements that are based on our management’s beliefs and assumptions and on information currently available to our management. These statements relate to future events or our future financial performance, and involve known and unknown risks, uncertainties and other Some of the key factors that maycould cause our actual results levels of activity, performance or achievements to be materially differentdiffer from any future results, levels of activity, performance or achievements expressed or implied by these forward-looking statements. Forward-looking statements include, but are not limited to, statements about:our expectations include:

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This prospectus and the documents incorporated by reference also contain estimates, projections and other information concerning our industry, our business and the markets for our solutions. Information that is based on estimates, forecasts, projections, market research or similar methodologies is inherently subject to uncertainties, and actual events or circumstances may differ materially from events and circumstances reflected in this information. Unless otherwise expressly stated, we obtained this industry, business, market and other data from reports, research surveys, studies and similar data prepared by market research firms and other third parties, industry and general publications, government data and similar sources.

You should read this prospectus and the documents that we incorporate by reference in this prospectus completely and with the understanding that our actual future results may be materially different from what we expect. The forward-looking statements in this prospectus and the documents we incorporate by reference herein represent our viewsExcept as of their respective dates. We anticipate that subsequent events and developments will cause our views to change. However, while we may elect to update these forward-looking statements at some point in the future, we have no current intention of doing so except to the extent required by applicable law. You should, therefore,law, we do not rely on theseplan to publicly update or revise any forward-looking statements contained herein, whether as representing our views asa result of any date subsequent to the date of this prospectus.new information, future events or otherwise.

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USE OF PROCEEDS

ThisWe intend to use the net proceeds from the sale of any securities offered under this prospectus for general corporate purposes unless otherwise indicated in the applicable prospectus supplement. General corporate purposes may include research and development expenditures, commercial expenditures, capital expenditures, working capital, the acquisition of companies or businesses and the repayment and refinancing of debt. We have not determined the amount of net proceeds to be used specifically for such purposes. As a result, management will retain broad discretion over the allocation of net proceeds. Pending the use of the net proceeds, we may invest the proceeds in interest-bearing, investment-grade securities, certificates of deposit or government securities. When we offer and sell the securities to which this prospectus relates, the prospectus supplement related to such offering will set forth our intended use of the potential resaleproceeds, if any, received from the sale of such securities.

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SECURITIES THAT MAY BE OFFERED

We may offer under this prospectus shares of our Common Stock and preferred stock, par value $0.001, or Preferred Stock, various series of debt securities, warrants to purchase any of such securities and units comprised of one or more of the other securities that may be offered under this prospectus, either individually or in combination with other securities, with a total value of up to $100,000,000 from time to time at prices and on terms to be determined at the time of any offering. This prospectus provides you with a general description of the securities we may offer. Each time we offer a type or series of securities under this prospectus, we will provide a prospectus supplement that will describe the specific amounts, prices and other important terms of the securities, including, to the extent applicable:

This prospectus may not be used to offer or sell securities unless accompanied by an applicable prospectus supplement. The prospectus supplement and any related free writing prospectus that we may authorize to be provided to you may also add, update or change information contained in this prospectus or in documents we have incorporated by reference. However, no prospectus supplement or free writing prospectus will offer a security that is not registered and described in this prospectus at the time of the effectiveness of the registration statement of which this prospectus is a part.

The following descriptions are not complete and may not contain all the information you should consider before investing in any securities we may offer hereunder; they are summarized from, and qualified by reference to, our restated certificate of incorporation, amended and restated by-laws, as amended, and the other documents referred to in the descriptions, all of which are or will be publicly filed with the SEC, as applicable. See “Where You Can Find More Information.”

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DESCRIPTION OF DEBT SECURITIES

We may offer debt securities which may be senior or subordinated. We refer to the senior debt securities and the subordinated debt securities collectively as debt securities. The following description summarizes the general terms and provisions of the debt securities. We will describe the specific terms of the debt securities and the extent, if any, to which the general provisions summarized below apply to any series of debt securities in the prospectus supplement relating to the series and any applicable free writing prospectus that we authorize to be delivered.

We may issue senior debt securities from time to time, in one or more series under a senior indenture to be entered into between us and a senior trustee to be named in a prospectus supplement, which we refer to as the senior trustee. We may issue subordinated debt securities from time to time, in one or more series under a subordinated indenture to be entered into between us and a subordinated trustee to be named in a prospectus supplement, which we refer to as the subordinated trustee. The forms of senior indenture and subordinated indenture are filed as exhibits to the registration statement of which this prospectus forms a part. Together, the senior indenture and the subordinated indenture are referred to as the indentures and, together, the senior trustee and the subordinated trustee are referred to as the trustees. This prospectus briefly outlines some of the provisions of the indentures. The following summary of the material provisions of the indentures is qualified in its entirety by the provisions of the indentures, including definitions of certain terms used in the indentures. Wherever we refer to particular sections or defined terms of the indentures, those sections or defined terms are incorporated by reference in this prospectus or the applicable prospectus supplement. You should review the indentures that are filed as exhibits to the registration statement of which this prospectus forms a part for additional information.

None of the indentures will limit the amount of debt securities that we may issue. The applicable indenture will provide that debt securities may be issued up to an aggregate principal amount authorized from time to time by us and may be payable in any currency or currency unit designated by us or in amounts determined by reference to an index.

General

The senior debt securities will constitute our unsecured and unsubordinated general obligations and will rank pari passu with our other unsecured and unsubordinated obligations. The subordinated debt securities will constitute our unsecured and subordinated general obligations and will be junior in right of payment to our senior indebtedness (including senior debt securities), as described under the heading “—Certain Terms of the Subordinated Debt Securities—Subordination.” The debt securities will be structurally subordinated to all existing and future indebtedness and other liabilities of our subsidiaries unless such subsidiaries expressly guarantee such debt securities.

The debt securities will be our unsecured obligations. Any secured debt or other secured obligations will be effectively senior to the debt securities to the extent of the value of the assets securing such debt or other obligations.

The applicable prospectus supplement and/or free writing prospectus will include any additional or different terms of the debt securities of any series being offered, including the following terms:

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When we refer to “principal” in this section with reference to the debt securities, we are also referring to “premium, if any.”

We may from time to time, without notice to or the consent of the holders of any series of debt securities, create and issue further debt securities of any such series ranking equally with the debt securities of such series in all respects (or in all respects other than (1) the payment of interest accruing prior to the issue date of such further debt securities or (2) the first payment of interest following the issue date of such further debt securities). Such further debt securities may be consolidated and form a single series with the debt securities of such series and have the same terms as to status, redemption or otherwise as the debt securities of such series.

You may present debt securities for exchange and you may present debt securities for transfer in the manner, at the places and subject to the restrictions set forth in the debt securities and the applicable prospectus supplement. We will provide you those services without charge, although you may have to pay any tax or other governmental charge payable in connection with any exchange or transfer, as set forth in the indenture.

Debt securities may bear interest at a fixed rate or a floating rate. Debt securities bearing no interest or interest at a rate that at the time of issuance is below the prevailing market rate (original issue discount securities) may be sold at a discount below their stated principal amount. U.S. federal income tax considerations applicable to any such discounted debt securities or to certain debt securities issued at par which are treated as having been issued at a discount for U.S. federal income tax purposes will be described in the applicable prospectus supplement.

We may issue debt securities with the principal amount payable on any principal payment date, or the amount of interest payable on any interest payment date, to be determined by reference to one or more currency

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exchange rates, securities or baskets of securities, commodity prices or indices. You may receive a payment of principal on any principal payment date, or a payment of interest on any interest payment date, that is greater than or less than the amount of principal or interest otherwise payable on such dates, depending on the value on such dates of the applicable currency, security or basket of securities, commodity or index. Information as to the methods for determining the amount of principal or interest payable on any date, the currencies, securities or baskets of securities, commodities or indices to which the amount payable on such date is linked and certain related tax considerations will be set forth in the applicable prospectus supplement.

Certain Terms of the Senior Debt Securities

Covenants. Unless we indicate otherwise in a prospectus supplement, the senior debt securities will not contain any financial or restrictive covenants, including covenants restricting either us or our subsidiaries from incurring, issuing, assuming or guaranteeing any indebtedness secured by a lien on any of our or our subsidiaries’ property or capital stock, or restricting either us or our subsidiaries from entering into sale and leaseback transactions.

Consolidation, Merger and Sale of Assets. Unless we indicate otherwise in a prospectus supplement, we may not consolidate with or merge into any other person, in a transaction in which we are not the surviving corporation, or convey, transfer or lease our properties and assets substantially as an entirety to any person, in either case, unless:

No Protection in the Event of a Change in Control. Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, the senior debt securities will not contain any provisions that may afford holders of the senior debt securities protection in the event we have a change in control or in the event of a highly leveraged transaction (whether or not such transaction results in a change in control).

Events of Default. Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, the following are events of default under the senior indenture for any series of senior debt securities:

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The default by us under any other debt, including any other series of debt securities, is not a default under the senior indenture.

If an event of default other than an event of default specified in the fourth bullet point above occurs with respect to a series of senior debt securities and is continuing under the senior indenture, then, and in each such case, either the trustee or the holders of not less than 25% in aggregate principal amount of such series then outstanding under the senior indenture (each such series voting as a separate class) by written notice to us and to the trustee, if such notice is given by the holders, may, and the trustee at the request of such holders shall, declare the principal amount of and accrued interest on such series of senior debt securities to be immediately due and payable, and upon this declaration, the same shall become immediately due and payable.

If an event of default specified in the fourth bullet point above occurs and is continuing, the entire principal amount of and accrued interest on each series of senior debt securities then outstanding shall become immediately due and payable.

Unless otherwise specified in the prospectus supplement relating to a series of senior debt securities originally issued at a discount, the amount due upon acceleration shall include only the original issue price of the senior debt securities, the amount of original issue discount accrued to the date of acceleration and accrued interest, if any.

Upon certain conditions, declarations of acceleration may be rescinded and annulled and past defaults may be waived by the holders of a majority in aggregate principal amount of all the senior debt securities of such series affected by the default, each series voting as a separate class. Furthermore, subject to various provisions in the senior indenture, the holders of a majority in aggregate principal amount of a series of senior debt securities, by notice to the trustee, may waive an existing default or event of default with respect to such senior debt securities and its consequences, except a default in the payment of principal of or interest on such senior debt securities or in respect of a covenant or provision of the senior indenture which cannot be modified or amended without the consent of the holders of each such senior debt security. Upon any such waiver, such default shall cease to exist, and any event of default with respect to such senior debt securities shall be deemed to have been cured, for every purpose of the senior indenture; but no such waiver shall extend to any subsequent or other default or event of default or impair any right consequent thereto.

The holders of a majority in aggregate principal amount of a series of senior debt securities may direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee with respect to such senior debt securities. However, the trustee may refuse to follow any direction that conflicts with law or the senior indenture, that may involve the trustee in personal liability or that the trustee determines in good faith may be unduly prejudicial to the rights of holders of such series of senior debt securities not joining in the giving of such direction and may take any other action it deems proper that is not inconsistent with any such direction received from holders of such series of senior debt securities. A holder may not pursue any remedy with respect to the senior indenture or any series of senior debt securities unless:

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These limitations, however, do not apply to the right of any holder of a senior debt security to receive payment of the principal of and interest on such senior debt security in accordance with the terms of such debt security, or to bring suit for the enforcement of any such payment in accordance with the terms of such debt security, on or after the due date for the senior debt securities, which right shall not be impaired or affected without the consent of the holder.

The senior indenture requires certain of our officers to certify, on or before a fixed date in each year in which any senior debt security is outstanding, as to their knowledge of our compliance with all covenants, agreements and conditions under the senior indenture.

Satisfaction and Discharge. We can satisfy and discharge our obligations to holders of any series of debt securities if:

Under current U.S. federal income tax law, the deposit and our legal release from the debt securities would be treated as though we exchanged your debt securities for your share of the cash and debt securities or bonds deposited in trust. In that event, you could recognize a gain or loss on the debt securities you exchange. Purchasers of the debt securities should consult their own advisers with respect to the tax consequences to them of such deposit and discharge, including the applicability and effect of tax laws other than the U.S. federal income tax law.

Defeasance. Unless the applicable prospectus supplement provides otherwise, the following discussion of legal defeasance and discharge and covenant defeasance will apply to any series of debt securities issued under the indentures.

Legal Defeasance. We can legally release ourselves from any payment or other obligations on the debt securities of any series (called “legal defeasance”) if certain conditions are met, including the following:

If we accomplish legal defeasance, as described above, you would have to rely solely on the trust deposit for repayment of the debt securities. You could not look to us for repayment in the event of any shortfall.

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Covenant Defeasance. Without any change of current U.S. federal tax law, we can make the same type of deposit described above and be released from some of the covenants in the debt securities (called “covenant defeasance”). In that event, you would lose the protection of those covenants but would gain the protection of having money and securities set aside in trust to repay the debt securities. In order to achieve covenant defeasance, we must do the following (among other things):

If we accomplish covenant defeasance, you could still look to us for repayment of the debt securities if there were a shortfall in the trust deposit. In fact, if one of the events of default occurred (such as our bankruptcy) and the debt securities become immediately due and payable, there may be such a shortfall. Depending on the events causing the default, you may not be able to obtain payment of the shortfall.

Modification and Waiver. We and the trustee may amend or supplement the senior indenture or the senior debt securities without the consent of any holder:

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Other amendments and modifications of the senior indenture or the senior debt securities issued may be made, and our compliance with any provision of the senior indenture with respect to any series of senior debt securities may be waived, with the consent of the holders of a majority of the aggregate principal amount of the outstanding senior debt securities of all series affected by the amendment or modification (voting together as a single class); provided, however, that each affected holder must consent to any modification, amendment or waiver that:

It shall not be necessary for the holders to approve the particular form of any proposed amendment, supplement or waiver, but it shall be sufficient if the holders’ consent approves the substance thereof. After an amendment, supplement or waiver of the senior indenture in accordance with the provisions described in this section becomes effective, the trustee must give to the holders affected thereby certain notice briefly describing the amendment, supplement or waiver. Any failure by the trustee to give such notice, or any defect therein, shall not, however, in any way impair or affect the validity of any such amendment, supplemental indenture or waiver.

No Personal Liability of Incorporators, Stockholders, Officers, Directors. The senior indenture provides that no recourse shall be had under any obligation, covenant or agreement of ours in the senior indenture or any supplemental indenture, or in any of the senior debt securities or because of the creation of any indebtedness represented thereby, against any of our incorporators, stockholders, officers or directors, past, present or future, or of any predecessor or successor entity thereof under any law, statute or constitutional provision or by the enforcement of any assessment or by any legal or equitable proceeding or otherwise. Each holder, by accepting the senior debt securities, waives and releases all such liability.

Concerning the Trustee. The senior indenture provides that, except during the continuance of an event of default, the trustee will not be liable except for the performance of such duties as are specifically set forth in the senior indenture. If an event of default has occurred and is continuing, the trustee will exercise such rights and powers vested in it under the senior indenture and will use the same degree of care and skill in its exercise as a prudent person would exercise under the circumstances in the conduct of such person’s own affairs.

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The senior indenture and the provisions of the Trust Indenture Act of 1939 incorporated by reference therein contain limitations on the rights of the trustee thereunder, should it become a creditor of ours or our subsidiaries, to obtain payment of claims in certain cases or to realize on certain property received by it in respect of any such claims, as security or otherwise. The trustee is permitted to engage in other transactions, provided that if it acquires any conflicting interest (as defined in the Trust Indenture Act), it must eliminate such conflict or resign.

We may have normal banking relationships with the senior trustee in the ordinary course of business.

Unclaimed Funds. All funds deposited with the trustee or any paying agent for the payment of principal, premium, interest or additional amounts in respect of the senior debt securities that remain unclaimed for two years after the date upon which such principal, premium or interest became due and payable will be repaid to us. Thereafter, any right of any holder of senior debt securities to such funds shall be enforceable only against us, and the trustee and paying agents will have no liability therefor.

Governing Law. The senior indenture and the senior debt securities will be governed by, and construed in accordance with, the internal laws of the State of New York.

Certain Terms of the Subordinated Debt Securities

Other than the terms of the subordinated indenture and subordinated debt securities relating to subordination or otherwise as described in the prospectus supplement relating to a particular series of subordinated debt securities, the terms of the subordinated indenture and subordinated debt securities are identical in all material respects to the terms of the senior indenture and senior debt securities.

Additional or different subordination terms may be specified in the prospectus supplement applicable to a particular series.

Subordination. The indebtedness evidenced by the subordinated debt securities is subordinate to the prior payment in full of all of our senior indebtedness, as defined in the subordinated indenture. During the continuance beyond any applicable grace period of any default in the payment of principal, premium, interest or any other payment due on any of our senior indebtedness, we may not make any payment of principal of or interest on the subordinated debt securities (except for certain sinking fund payments). In addition, upon any payment or distribution of our assets upon any dissolution, winding-up, liquidation or reorganization, the payment of the principal of and interest on the subordinated debt securities will be subordinated to the extent provided in the subordinated indenture in right of payment to the prior payment in full of all our senior indebtedness. Because of this subordination, if we dissolve or otherwise liquidate, holders of our subordinated debt securities may receive less, ratably, than holders of our senior indebtedness. The subordination provisions do not prevent the occurrence of an event of default under the subordinated indenture.

The term “senior indebtedness” of a person means with respect to such person the principal of, premium, if any, interest on, and any other payment due pursuant to any of the following, whether outstanding on the date of the subordinated indenture or incurred by that person in the future:

The selling stockholders will receive

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unless, in the case of any particular indebtedness, renewal, extension or refunding, the instrument creating or evidencing it or the assumption or guarantee relating to it expressly provides that such as commissions and discounts, if any, attributableindebtedness, renewal, extension or refunding is not superior in right of payment to the sale or dispositionsubordinated debt securities. Our senior debt securities constitute senior indebtedness for purposes of the shares will be borne by the selling stockholders.subordinated debt indenture.

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DESCRIPTION OF CAPITAL STOCK

The following description of our capital stock is intended as a summary only and therefore is not a complete description of our capital stock. This description is based upon, and is qualified by reference to, our amended and restated certificate of incorporation, our amended and restated by-laws, and applicable provisions of Delaware corporate law. You should read our amended and restated certificate of incorporation and our amended and restated by-laws, in each case, as amended and supplemented, which are filed as exhibits to the registration statement of which this prospectus forms a part, for the provisions that are important to you.

Our authorized capital stock consists of 500,000,000 shares of common stockCommon Stock and 5,000,000 shares of preferred stock, par value $0.001 per share.Preferred Stock. Our common stockCommon Stock is registered under Section 12(b) of the Exchange Act. As of July 11,June 30, 2023, we had 97,012,816 shares of common stockCommon Stock outstanding.

Common Stock

Voting Rights. Holders of our common stockCommon Stock are entitled to one vote for each share held on all matters submitted to a vote of stockholders and do not have cumulative voting rights. Any matter other than the election of directors to be voted upon by the stockholders at any meeting at which a quorum is present shall be decided by the vote of the holders of shares of stock having a majority in voting power of the votes cast by the holders of all of the shares of stock present or represented at the meeting and voting affirmatively or negatively on the matter, except when a different vote is required by law, our amendedamened and restated certificate of incorporation or our amended and restated by-laws. An election of directors by our stockholders shall be determined by a plurality of the votes cast by the stockholders entitled to vote on the election.

Dividends. Holders of common stockCommon Stock are entitled to receive proportionately any dividends as may be declared by our board of directors, subject to any preferential dividend rights of any series of preferred stockPreferred Stock that we may designate and issue in the future.

Liquidation, Dissolution and Winding Up. In the event of our liquidation or dissolution, the holders of common stockCommon Stock are entitled to receive proportionately our net assets available for distribution to stockholders after the payment of all debts and other liabilities and subject to the prior rights of any outstanding preferred stock.Preferred Stock.

Other Rights. Holders of common stockCommon Stock have no preemptive, subscription, redemption or conversion rights and there are no sinking fund provisions applicable to our common stock.Common Stock. The rights, preferences and privileges of holders of common stockCommon Stock are subject to and may be adversely affected by the rights of the holders of shares of any series of preferred stockPreferred Stock that we may designate and issue in the future.

All outstanding shares are fully paid and non-assessable. When we issue shares of common stockCommon Stock under this prospectus, the shares will fully be paid and non-assessable and will not have, or be subject to, any preemptive or similar rights.

Listing on theThe Nasdaq Global Select Market

Our common stockCommon Stock is listed on theThe Nasdaq Global Select Market under the symbol “CASA.”

Authorized but Unissued Shares

The authorized but unissued shares of common stockCommon Stock are available for future issuance without stockholder approval, subject to any limitations imposed by the Nasdaq Listing Rules. These additional shares may be used for a variety of corporate finance transactions, acquisitions and employee benefit plans. The existence of authorized but unissued and unreserved common stockCommon Stock could make it more difficult or discourage an attempt to obtain control of us by means of a proxy contest, tender offer, merger or otherwise.

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Transfer Agent and Registrar

The transfer agent and registrar for our common stockCommon Stock is American Stock Transfer & Trust Company, LLC. The transfer agent and registrar’s address is 6201 15th Avenue, Brooklyn, New York, 11219, and its telephone number is (781) 921-8300.

Preferred Stock

As of June 30, 2023, no shares of our Preferred Stock were outstanding. The specific terms of any series of Preferred Stock will be described in the prospectus supplement relating to that series of Preferred Stock. The terms of any series of Preferred Stock may differ from the terms described below. Certain provisions of the Preferred Stock described below and in any applicable prospectus supplement are not complete.

We are authorized to issue “blank check” Preferred Stock, which may be issued in one or more series upon authorization of our board of directors. Our board of directors is authorized to fix the designation of the series, the number of authorized shares of the series, dividend rights and terms, conversion rights, voting rights, redemption rights and terms, liquidation preferences and any other rights, powers, preferences and limitations applicable to each series of Preferred Stock. The authorized shares of our Preferred Stock are available for issuance without further action by our stockholders, unless such action is required by applicable law or the rules of any stock exchange on which our securities may be listed. If the approval of our stockholders to designate and issue up to an aggregateis not required for the issuance of 5,000,000 shares of our Preferred Stock, our board of directors may determine not to seek stockholder approval.

A series of our Preferred Stock could, depending on the terms of such series, impede the completion of a merger, tender offer or other takeover attempt. Our board of directors will make any determination to issue such shares based upon its judgment as to the best interests of our stockholders. Our directors, in so acting, could issue preferred stock having terms that could discourage an acquisition attempt through which an acquirer may be able to change the composition of our board of directors, including a tender offer or other transaction that some, or a majority, of our stockholders might believe to be in onetheir best interests or morein which stockholders might receive a premium for their stock over the then-current market price of the stock.

The Preferred Stock has the terms described below unless otherwise provided in the prospectus supplement relating to a particular series of Preferred Stock. You should read the prospectus supplement relating to the particular series of Preferred Stock being offered for specific terms, including:

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The Preferred Stock will, when issued, be fully paid and nonassessable. Unless otherwise specified in the prospectus supplement, each series of Preferred Stock will rank equally as to dividends and liquidation rights in all respects with each other series of Preferred Stock. The rights of holders of shares of each series of Preferred Stock will be subordinate to those of our general creditors.

Rank. Unless otherwise specified in the prospectus supplement, the Preferred Stock will, with respect to dividend rights and rights upon our liquidation, dissolution or winding up of our affairs, rank:

The term “equity securities” does not include convertible debt securities.

Dividends. Holders of the Preferred Stock of each series will be entitled to receive, when, as and if declared by our board of directors, cash dividends at such rates and on such dates described in the prospectus supplement. Different series of Preferred Stock may be entitled to dividends at different rates or based on different methods of calculation. The dividend rate may be fixed or variable or both. Dividends will be payable to the holders of record as they appear on our stock books on record dates fixed by our board of directors, as specified in the applicable prospectus supplement.

Dividends on any series of Preferred Stock may be cumulative or noncumulative, as described in the applicable prospectus supplement. If our board of directors does not declare a dividend payable on a dividend payment date on any series of noncumulative Preferred Stock, then the holders of that noncumulative Preferred Stock will have no right to receive a dividend for that dividend payment date, and we will have no obligation to pay the dividend accrued for that period, whether or not dividends on that series are declared payable on any future dividend payment dates. Dividends on any series of cumulative Preferred Stock will accrue from the date we initially issue shares of such series or such other date specified in the applicable prospectus supplement.

No dividends may be declared or paid or funds set apart for the payment of any dividends on any parity securities unless full dividends have been paid or set apart for payment on the Preferred Stock. If full dividends are not paid, the Preferred Stock will share dividends pro rata with the parity securities.

No dividends may be declared or paid or funds set apart for the payment of dividends on any junior securities unless full dividends for all dividend periods terminating on or prior to the date of the declaration or payment will have been paid or declared and a sum sufficient for the payment set apart for payment on the Preferred Stock.

Liquidation Preference. Upon any voluntary or involuntary liquidation, dissolution or winding up of our affairs, then, before we make any distribution or payment to the holders of any Common Stock or any other class or series of our capital stock ranking junior to the Preferred Stock in the distribution of assets upon any liquidation, dissolution or winding up of our affairs, the holders of each series of Preferred Stock shall be entitled to receive out of assets legally available for distribution to stockholders, liquidating distributions in the amount of the liquidation preference per share set forth in the prospectus supplement, plus any accrued and unpaid dividends thereon. Such dividends will not include any accumulation in respect of unpaid noncumulative dividends for prior dividend periods. Unless otherwise specified in the prospectus supplement, after payment of

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the full amount of their liquidating distributions, the holders of Preferred Stock will have no right or claim to any of our remaining assets. Upon any such voluntary or involuntary liquidation, dissolution or winding up, if our available assets are insufficient to pay the amount of the liquidating distributions on all outstanding Preferred Stock and the corresponding amounts payable on all other classes or series of our capital stock ranking on parity with the Preferred Stock and all other such classes or series of shares of capital stock ranking on parity with the Preferred Stock in the distribution of assets, then the holders of the Preferred Stock and all other such classes or series of capital stock will share ratably in any such distribution of assets in proportion to the full liquidating distributions to which they would otherwise be entitled.

Upon any such liquidation, dissolution or winding up and if we have made liquidating distributions in full to all holders of Preferred Stock, we will distribute our remaining assets among the holders of any other classes or series of capital stock ranking junior to the Preferred Stock according to their respective rights and preferences and, in each case, according to their respective number of shares. For such purposes, our consolidation or merger with or into any other corporation, trust or entity, or the sale, lease or conveyance of all or substantially all of which mayour property or assets will not be greater than the rightsdeemed to constitute a liquidation, dissolution or winding up of our common stock. Our boardaffairs.

Redemption. If so provided in the applicable prospectus supplement, the Preferred Stock will be subject to mandatory redemption or redemption at our option, as a whole or in part, in each case upon the terms, at the times and at the redemption prices set forth in such prospectus supplement.

The prospectus supplement relating to a series of directorsPreferred Stock that is subject to mandatory redemption will specify the number of shares of Preferred Stock that shall be redeemed by us in each year commencing after a date to be specified, at a redemption price per share to be specified, together with an amount equal to all accrued and unpaid dividends thereon to the date of redemption. Unless the shares have a cumulative dividend, such accrued dividends will not include any accumulation in respect of unpaid dividends for prior dividend periods. We may authorizepay the redemption price in cash or other property, as specified in the applicable prospectus supplement. If the redemption price for Preferred Stock of any series is payable only from the net proceeds of the issuance of preferredshares of our capital stock, with votingthe terms of such Preferred Stock may provide that, if no such shares of our capital stock shall have been issued or to the extent the net proceeds from any issuance are insufficient to pay in full the aggregate redemption price then due, such Preferred Stock shall automatically and mandatorily be converted into the applicable shares of our capital stock pursuant to conversion provisions specified in the applicable prospectus supplement.

Notwithstanding the foregoing, we will not redeem any Preferred Stock of a series unless:

In addition, we will not acquire any Preferred Stock of a series unless:

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However, at any time we may purchase or acquire Preferred Stock of that series (1) pursuant to a purchase or exchange offer made on the same terms to holders of all outstanding Preferred Stock of such series or (2) by conversion into or exchange for shares of our capital stock ranking junior to the Preferred Stock of such series as to dividends and upon liquidation.

If fewer than all of the outstanding shares of Preferred Stock of any series are to be redeemed, we will determine the number of shares that may be redeemed pro rata from the holders of record of such shares in proportion to the number of such shares held or for which redemption is requested by such holder or by any other equitable manner that we determine. Such determination will reflect adjustments to avoid redemption of fractional shares.

Unless otherwise specified in the prospectus supplement, we will mail notice of redemption at least 30 days but not more than 60 days before the redemption date to each holder of record of Preferred Stock to be redeemed at the address shown on our stock transfer books. Each notice shall state:

If notice of redemption has been given and we have set aside the funds necessary for such redemption in trust for the benefit of the holders of any shares called for redemption, then from and after the redemption date, dividends will cease to accrue on such shares, and all rights of the holders of such shares will terminate, except the right to receive the redemption price.

Voting Rights. Holders of Preferred Stock will not have any voting rights, except as required by law or as indicated in the applicable prospectus supplement.

Unless otherwise provided for under the terms of any series of Preferred Stock, no consent or vote of the holders of shares of Preferred Stock or any series thereof shall be required for any amendment to our common stockrestated certificate of incorporation that would increase the number of authorized shares of Preferred Stock or the number of authorized shares of any series thereof or decrease the number of authorized shares of Preferred Stock or the number of authorized shares of any series thereof (but not below the number of authorized shares of Preferred Stock or such series, as the case may be, then outstanding).

Conversion Rights. The terms and conditions, if any, upon which any series of Preferred Stock is convertible into our Common Stock will be set forth in the likelihood that such holdersapplicable prospectus supplement relating thereto. Such terms will receive dividend payments and payments upon our liquidation.

The purpose of authorizing our board of directors to issue preferred stock in one or more series and determineinclude the number of shares of Common Stock into which the shares of Preferred Stock are convertible, the conversion price, rate or manner of calculation thereof, the conversion period, provisions as to whether conversion will be at our option or at the option of the holders of the Preferred Stock, the events requiring an adjustment of the conversion price and provisions affecting conversion in the seriesevent of the redemption.

Transfer Agent and its rights, preferences, privilegesRegistrar. The transfer agent and restrictions is to eliminate delays associated with a stockholder vote on specific issuances. The issuance of preferred stock, while providing flexibility in connection with possible future financings and acquisitions and other corporate purposes, could, under certain circumstances, haveregistrar for the effect of delaying, deferring or preventing a change in control of our company, as further discussed below under “Anti-Takeover Effects of Delaware Law and Our Charter and By-laws.”

When we issue shares of preferred stock, the sharesPreferred Stock will be fully paid and non-assessable and will not be subject to any preemptive or similar rightsset forth in the applicable prospectus supplement.

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Common Stock Registration Rights

Pursuant to the terms of that certain registration rights agreement, dated as of April 26, 2010, or, the 2010 registration rights agreement, certain of our stockholders are entitled to rights with respect to the registration of their shares under the Securities Act until the earliest of the seventh (7th) anniversary of our initial public offering, or until no registrable securities could remain outstanding. We refer to these shares collectively as registrable securities.

Demand Registration RightsRights.

Four As of August 4, 2023, four holders of an aggregate of 33,874,480 shares of our common stockCommon Stock are entitled to demand registration rights. Under the terms of the 2010 registration rights agreement, we will be required, upon the written request of any holder to register at least 20% of the securities eligible for registration then outstanding and held by such holder, or such lesser percentage that would result in an aggregate offering price of at least $5.0 million, to file a registration statement and use commercially reasonable efforts to effect the registration of all or a portion of these shares for public resale. We are required to effect only two registrations for each holder pursuant to this provision of the 2010 registration rights agreement.

Short-Form Registration RightsRights.

Pursuant to the 2010 registration rights agreement, if we are eligible to file a registration statement on Form S-3, upon the written request of one or more holders to sell registrable securities at an aggregate price of at least $5.0 million, we will be required to use our best efforts to effect a registration of such shares. We are not required to effect more than two registrations in any twelve-month period pursuant to this provision of the 2010 registration rights agreement. The right to have such shares registered on Form S-3 is further subject to other specified conditions and limitations.

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under our then-existing credit facility, and JPMorgan Chase Bank, N.A. Pursuant to the exchange agreement, we purchased and assumed 100% of each participating lender’s portion of our outstanding term loan debt, or the purchased loan, in exchange for, among other things:

Warrant Agreement

On June 15, 2023, we entered into the Warrant Agreement with American Stock Transfer & Trust Company, LLC, as warrant agent. Subject to the terms and conditions set forth therein, each participating lender (or its affiliates or designees) has received (or has the right to receive) warrants, or the participating lender warrants, with an exercise price of $0.01 per share of common stockCommon Stock on a pro rata basis in accordance with the amount of the superpriority term loans held by such participating lender. The participating lender warrants issued or issuable pursuant to the Warrant Agreement vest in three tranches: the first tranche, which comprises an aggregate of 10% of the shares of our common stockCommon Stock issued and outstanding as of the closing of the superpriority credit agreement, or the closing date, is immediately exercisable upon issuance of the participating lender warrants; the second tranche, which comprises an aggregate of 5% of the shares of our common stockCommon Stock issued and

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written demand of any participating lender, we must effect the registration of any warrant shares not covered by an effective resale registration statement previously filed by us. Additionally, certain of the participating lenders have a right to be issued a participating lender warrant at their election, which we refer to as springing warrants. ThereAs of August 4, 2023, there are 4,943,060 warrant shares underlying springing warrants issuable to three participating lenders that have not yet been issued. Under the terms of the 2023 registration rights agreement, we will be required, upon the written request of any participating lender to effect the registration of all or a portion of the warrant shares underlying such springing warrants within 30 days of receipt of notice from such participating lenders that they have exercised their right to be issued a springing warrant, provided, however, that (a) we will not be obligated to file more than nine registration statements with respect to the warrant shares underlying the springing warrants, (b) we are not obligated to file more than three such registrations statements during any calendar year, (c) we are not obligated to file a registration statement registering less than an aggregate of 50,000 warrant shares and (d) each institutional lender that is a participating lender (including all of such lender’s affiliated funds that may also be participating lenders) is not permitted to make more than one demand for registration of their warrant shares in any calendar year. Additionally, within 30 days immediately following the written demand of any participating lender, we must effect the registration of any warrant shares not covered by an effective resale registration statement previously filed by us.

Piggyback Registration RightsRights.

Pursuant to the 2023 registration rights agreement, if we register any of our securities either for our own account or for the account of other security holders, theas of August 4, 2023, participating lenders that hold warrants or springing warrants representing an aggregate of 7,465,261 warrants shares are entitled to include their shares in the registration, provided however, that the managing underwriters may limit the number of warrant shares included in such underwritten offering to the number of shares that the managing underwriters advise us will not jeopardize the success of the offering, and provided further, that the participating lenders shall not be permitted to include the warrant shares in such registration statement unless all of the securities registered for our own account and any registrable securities being registered pursuant to the exercise of piggyback rights under the 2010 registration rights agreement have been included in such registration.

Anti-Takeover Effects of Delaware Law and Our Charter and By-laws

Delaware Law. We are subject to Section 203 of the Delaware General Corporation Law, or the DGCL. Subject to certain exceptions, Section 203 prevents a publicly held Delaware corporation from engaging in a “business combination” with any “interested stockholder” for three years following the date that the person became an interested stockholder, unless the interested stockholder attained such status with the approval of our board of directors or unless the business combination is approved in a prescribed manner. A “business combination”

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Staggered Board; Removal of Directors. Our amended and restated certificate of incorporation and our amended and restated by-laws divide our board of directors into three classes with staggered three-year terms. In addition, a director may be removed only for cause and only by the affirmative vote of the holders of at least 75% of the votes that all our stockholders would be entitled to cast in an election of directors. Any vacancy on our board of directors, including a vacancy resulting from an enlargement of our board of directors, may be filled only by vote of a majority of our directors then in office. The classification of our board of directors and the limitations on the removal of directors and filling of vacancies could make it more difficult for a third party to acquire, or discourage a third party from seeking to acquire, control of our company.

Supermajority Voting. The DGCL provides generally that the affirmative vote of a majority of the shares entitled to vote on any matter is required to amend a corporation’s certificate of incorporation or bylaws, unless a corporation’s certificate of incorporation or bylaws, as the case may be, requires a greater percentage. Our amended and restated by-laws may be amended or repealed by a majority vote of our board of directors or the affirmative vote of the holders of at least 75% of the votes that all our stockholders would be entitled to cast in an election of directors. In addition, the affirmative vote of the holders of at least 75% of the votes which all our stockholders would be entitled to cast in an election of directors is required to amend, repeal, or adopt any provisions inconsistent with, any of the provisions of our amended and restated certificatethe Certificate of incorporation.Incorporation described above.

Stockholder Action; Special Meeting of Stockholders; Advance Notice Requirements for Stockholder Proposals and Director Nominations. Our amended and restated certificate of incorporation provides that any action required or permitted to be taken by our stockholders must be effected at a duly called annual or special meeting of such stockholders and may not be effected by any consent in writing by such stockholders. Our amended and restated certificate of incorporation and our amended and restated by-laws also provide that, except as otherwise required by law, special meetings of our stockholders can only be called by our board of directors. In addition, our amended and restated by-laws establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of stockholders, including proposed nominations of candidates for election to our board of directors. Stockholders at an annual meeting may only consider proposals or nominations specified in the notice of meeting or brought before the meeting by or at the direction of our board of directors, or by a stockholder of record on the record date for the meeting, who is entitled to vote at the meeting and who has delivered timely written notice in proper form to our secretary of the stockholder’s intention to bring such business before the meeting. These provisions could have the effect of delaying until the next stockholder meeting stockholder actions that are favored by the holders of a majority of our outstanding voting securities. These provisions also could discourage a third party from making a tender offer for our common stock,Common Stock, because even if such third party acquired a majority of our outstanding voting stock, it would be able to take action as a stockholder, such as electing new directors or approving a merger, only at a duly called stockholders meeting and not by written consent.

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SELLING STOCKHOLDERSDESCRIPTION OF UNITS

ThisWe may issue units comprised of one or more of the other securities that may be offered under this prospectus, relatesin any combination. The following, together with the additional information we may include in the applicable prospectus supplement, summarizes the material terms and provisions of the units that we may offer under this prospectus. While the terms summarized below will apply generally to any units we may offer, we will describe the particular terms of any series of units in more detail in the applicable prospectus supplement.

Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately at any time, or at any time before a specified date.

Any applicable prospectus supplement will describe:

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DESCRIPTION OF WARRANTS

We may issue warrants to purchase Common Stock, Preferred Stock or debt securities. We may offer warrants separately or together with one or more additional warrants, Common Stock, Preferred Stock or debt securities, or any combination of those securities in the form of units, as described in the applicable prospectus supplement. If we issue warrants as part of a unit, the accompanying prospectus supplement will specify whether those warrants may be separated from the other securities in the unit prior to the expiration date of the warrants. The applicable prospectus supplement will also describe the following terms of any warrants:

The information presented regarding the selling stockholders is based, in part, on information the selling stockholders provided to us specifically for use in this prospectus. Each selling stockholder may sell all, some or none of its shares in this offering. See “Plan of Distribution.” We may supplement this prospectus from time to time in the future to update or change the selling stockholders list and

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FORMS OF SECURITIES

Each debt security, unit and warrant will be represented either by a certificate issued in definitive form to a particular investor or by one or more global securities representing the entire issuance of securities. Unless the applicable prospectus supplement provides otherwise, certificated securities in definitive form and global securities will be issued in registered form. Definitive securities name you or your nominee as the owner of the security, and in order to transfer or exchange these securities or to receive payments other than interest or other interim payments, you or your nominee must physically deliver the securities to the trustee, registrar, paying agent or other agent, as applicable. Global securities name a depositary or its nominee as the owner of the debt securities, units or warrants represented by these global securities. The depositary maintains a computerized system that will reflect each investor’s beneficial ownership of the securities through an account maintained by the selling stockholders. The registration for resaleinvestor with its broker/dealer, bank, trust company or other representative, as we explain more fully below.

Global Securities

We may issue the debt securities, units and warrants in the form of one or more fully registered global securities that will be deposited with a depositary or its nominee identified in the applicable prospectus supplement and registered in the name of that depositary or nominee. In those cases, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the sharesaggregate principal or face amount of common stock doesthe securities to be represented by global securities. Unless and until it is exchanged in whole for securities in definitive registered form, a global security may not necessarily meanbe transferred except as a whole by and among the depositary for the global security, the nominees of the depositary or any successors of the depositary or those nominees.

If not described below, any specific terms of the depositary arrangement with respect to any securities to be represented by a global security will be described in the prospectus supplement relating to those securities. We anticipate that the selling stockholdersfollowing provisions will sellapply to all depositary arrangements.

Ownership of beneficial interests in a global security will be limited to persons, called participants, that have accounts with the depositary or anypersons that may hold interests through participants. Upon the issuance of these shares. In addition,a global security, the selling stockholders may have sold, transferreddepositary will credit, on its book-entry registration and transfer system, the participants’ accounts with the respective principal or otherwise disposed of, or may sell, transfer or otherwise dispose of, at any time and from time to time, shares of common stock in transactions exempt from the registration requirementsface amounts of the Securities Act, after the date on which it provided the information set forth in the table below.

The following table sets forth, as of the date of this prospectus, the names of the selling stockholders, the aggregate number of shares of common stock beneficially owned, the aggregate number of shares of common stock that the selling stockholders may offer pursuant to this prospectus and the number of shares of common stocksecurities beneficially owned by the selling stockholders afterparticipants. Any dealers, underwriters or agents participating in the saledistribution of the securities offered hereby. Becausewill designate the selling stockholders are not obligatedaccounts to sellbe credited. Ownership of beneficial interests in a global security will be shown on, and the sharestransfer of common stock, we cannot estimateownership interests will be effected only through, records maintained by the amountdepositary, with respect to interests of sharesparticipants, and on the records of common stockparticipants, with respect to interests of persons holding through participants. The laws of some states may require that some purchasers of securities take physical delivery of these securities in definitive form. These laws may impair your ability to own, transfer or pledge beneficial interests in global securities.

So long as the selling stockholdersdepositary, or its nominee, is the registered owner of a global security, that depositary or its nominee, as the case may be, will hold upon consummation of any such sales. For purposesbe considered the sole owner or holder of the following table, wesecurities represented by the global security for all purposes under the applicable indenture, warrant agreement or unit agreement. Except as described below, owners of beneficial interests in a global security will not be entitled to have assumed the sale of allsecurities represented by the global security registered in their names, will not receive or be entitled to receive physical delivery of the sharessecurities in definitive form and will not be considered the owners or holders of common stock that may be offered for sale pursuant to this prospectus. Percentage of beneficial ownership is based on 97,012,816 shares of common stock outstanding as of July 19, 2023. Pursuant to Rule 416the securities under the Securities Act,applicable indenture, deposit agreement, purchase contract, unit agreement or warrant agreement. Accordingly, each person owning a beneficial interest in a global security must rely on the prospectus also coversprocedures of the depositary for that global security and, if that person is not a participant, on the procedures of the participant through which the person owns its interest, to exercise any additional shares of our common stock that may become issuable in connection with shares of common stock by reasonrights of a stock dividend, stock splitholder under the applicable indenture, warrant agreement or other similar transaction effected without our receivingunit agreement. We understand that under existing industry practices, if we request any cashaction of holders or other value, which resultsif an owner of a beneficial interest in an increase ina global security desires to give or take any action that a holder is entitled to give or take under the number of shares of our common stock outstanding.applicable indenture, warrant agreement or unit agreement, the

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depositary for the global security would authorize the participants holding the relevant beneficial interests to give or take that action, and the participants would authorize beneficial owners owning through them to give or take that action or would otherwise act upon the instructions of beneficial owners holding through them.

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PLAN OF DISTRIBUTION

The selling stockholders,We may sell securities:

We may also sell equity securities covered by this registration statement in an “at the market offering” as defined in Rule 415 under the Securities Act. Such offering may be made into an existing trading market for such securities in transactions at other than a fixed price, either:

In addition, we may issue the securities as a dividend or distribution or in a subscription rights offering to our existing security holders. This prospectus may be used herein includes donees, pledgees, transfereesin connection with any offering of our securities through any of these methods or other successors-in-interest selling sharesmethods described in the applicable prospectus supplement.

We may directly solicit offers to purchase securities, or agents may be designated to solicit such offers. We will, in the prospectus supplement relating to such offering, name any agent that could be viewed as an underwriter under the Securities Act, and describe any commissions that we must pay. Any such agent will be acting on a best efforts basis for the period of common stockits appointment or, interestsif indicated in sharesthe applicable prospectus supplement, on a firm commitment basis.

The distribution of common stock received after the date of this prospectus from a selling stockholder as a gift, pledge, partnership distribution or other transfer,securities may be effected from time to time sell, transferin one or otherwise disposemore transactions:

Each prospectus supplement will describe the method of distribution of the securities and any applicable restrictions.

The prospectus supplement with respect to the securities of a particular series will describe the terms of the offering of the securities, including the following:

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If any underwriters or agents are tradedutilized in the sale of the securities in respect of which this prospectus is delivered, we will enter into an underwriting agreement or in private transactions. These dispositions may be at fixed prices, at prevailing market pricesother agreement with them at the time of sale at prices related to them, and we will set forth in the prevailing market price, at varying prices determined atprospectus supplement relating to such offering the time of sale, or at negotiated prices.

The selling stockholders may use any one or morenames of the following methods when disposing of sharesunderwriters or interests therein:

The selling stockholders may, from time to time, pledge or grant aour existing security interest in some or all of the shares of common stock owned by them and, if they default in the performance of their secured obligations, the pledgees or secured parties may offer and sell the shares of common stock, from time to time, under this prospectus, or under an amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act of 1933, as amended (the “Securities Act”) amending the list of selling stockholders to include the pledgee, transferee or other successors in interest as selling stockholders under this prospectus. The selling stockholders also may transfer the shares of common stock in other circumstances, in which case the transferees, pledgees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.

In connection with the sale of our common stock or interests therein, the selling stockholdersholders, we may enter into hedging transactionsa standby underwriting agreement with broker-dealers or other financial institutions, whichdealers, acting as standby underwriters. We may in turn engage in short sales ofpay the common stock instandby underwriters a commitment fee for the course of hedging the positionssecurities they assume. The selling stockholders may also sell shares of our common stock short and deliver these securitiescommit to close out their short positions, or loan or pledge the common stock to broker-dealers that in turn may sell these securities. The selling stockholders may alsopurchase on a standby basis. If we do not enter into option ora standby underwriting arrangement, we may retain a dealer-manager to manage a subscription rights offering for us.

Remarketing firms, agents, underwriters, dealers and other transactions with broker-dealers or other financial institutions or the creation of one or more derivative securities which require the delivery to such broker-dealer or other financial institution of shares offered by this prospectus, which shares such broker-dealer or other financial institution may resell pursuant to this prospectus (as supplemented or amended to reflect such transaction).

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Under Rule 15c6-1 of the Exchange Act, trades in the secondary market generally are required to settle in two business days, unless the parties to any such trade expressly agree otherwise. The applicable prospectus supplement may provide that the original issue date for your securities may be more than two scheduled business days after the trade date for your securities. Accordingly, in such a case, if you wish to trade securities on any date prior to the registrationsecond business day before the original issue date for your securities, you will be required, by virtue of the shares offered by this prospectus.fact that your securities initially are expected to settle more than two scheduled business days after the trade date for your securities, to make alternative settlement arrangements to prevent a failed settlement.

The securities may be new issues of securities and may have no established trading market. The securities may or may not be listed on a national securities exchange. We have agreedcan make no assurance as to the liquidity of or the existence of trading markets for any of the securities.

In compliance with the selling stockholdersguidelines of the Financial Industry Regulatory Authority, or FINRA, the aggregate maximum discount, commission or agency fees or other items constituting underwriting compensation to keepbe received by any FINRA member or independent broker-dealer will not exceed 8% of the registration statement of whichproceeds from any offering pursuant to this prospectus constitutes a part effective until such time as all of the shares covered by thisand any applicable prospectus have been disposed of pursuant to and in accordance with the registration statement.supplement.

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LEGAL MATTERS

Unless indicatedthe applicable prospectus supplement indicates otherwise, the validity of the common stock being offered bysecurities in respect of which this prospectus has beenis being delivered will be passed upon for us by Goodwin Procter LLP, Boston, Massachusetts.

EXPERTS

The consolidated financial statements of Casa Systems, Inc. appearing in Casa Systems, Inc.’s Annual Report (Form 10-K) for the year ended December 31, 2022, and the effectiveness of Casa Systems, Inc.’s internal control over financial reporting as of December 31, 2022, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in its reports thereon (which contains an explanatory paragraph describing conditions that raise substantial doubt about the Company’s ability to continue as a going concern as described in Note 1 to the consolidated financial statements), which conclude, among other things, that Casa Systems, Inc. did not maintain effective internal control over financial reporting as of December 31, 2022, based on Internal Control—Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 Framework), because of the effects of the material weaknesses described therein, included therein, and incorporated herein by reference. Such financial statements have been incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

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CASA SYSTEMS, INC.

$100,000,000

Debt Securities

Common Stock

Preferred Stock

Units

Warrants

PROSPECTUS

                    , 2023

The information in this preliminary prospectus supplement is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus supplement is not an offer to sell these securities and we are not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to completion, dated August 8, 2023

PROSPECTUS SUPPLEMENT

CASA SYSTEMS, INC.

Up to $10,000,000

Common Stock

We have entered into an Equity Distribution Agreement, or the sale agreement, with Piper Sandler & Co., or Piper Sandler, dated August 8, 2023, relating to the sale of shares of our common stock, par value $0.001 per share, or Common Stock, offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the sale agreement, under this prospectus supplement we may offer and sell shares of our Common Stock having an aggregate offering price of up to $10,000,000 from time to time through Piper Sandler, acting as our agent.

Our Common Stock is listed on The Nasdaq Global Select Market under the symbol “CASA.” On August 4, 2023, the closing sale price of our Common Stock on The Nasdaq Global Select Market was $1.06 per share.

As of August 4, 2023, the aggregate market value of our outstanding Common Stock held by non-affiliates, or the public float, was $33.7 million, which was calculated based on 31,827,058 shares of our outstanding Common Stock held by non-affiliates at a price of $1.06 per share, the closing price of our Common Stock on such date. Pursuant to General Instruction I.B.6 of Form S-3, in no event will we sell shares pursuant to this Prospectus Supplement with a value of more than one-third of the aggregate market value of our Common Stock held by non-affiliates in any 12-month period, so long as the aggregate market value of our Common Stock held by non-affiliates is less than $75.0 million. During the 12 calendar months prior to, and including, the date of this Prospectus Supplement, we have not sold any securities pursuant to General Instruction I.B.6 of Form S-3.

We are a smaller reporting company as defined under the federal securities laws and, as such, have elected to comply with certain reduced public company reporting requirements for this prospectus supplement and future filings. See “Prospectus Supplement Summary—Implications of Being a Smaller Reporting Company.”

Upon delivery of a placement notice, and subject to our instructions in that notice and the terms and conditions of the sale agreement, Piper Sandler may sell our Common Stock under this prospectus supplement and the accompanying prospectus by any method permitted by law that is deemed an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended, or the Securities Act, including sales made directly on or through The Nasdaq Global Select Market, the existing trading market for our Common Stock. Piper Sandler is not required to sell any specific amount, but will act as our sales agent using commercially reasonable efforts consistent with its normal trading and sales practices. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

Piper Sandler will be entitled to compensation at a commission rate of 3.0% of the gross sales price per share sold under the sale agreement. See “Plan of Distribution” beginning on page S-19 for additional information regarding the compensation to be paid to Piper Sandler. In connection with the sale of our Common Stock on our behalf, Piper Sandler will be deemed to be an “underwriter” within the meaning of the Securities Act and the compensation of Piper Sandler will be deemed to be underwriting commissions or discounts. We have also agreed to provide indemnification and contribution to Piper Sandler with respect to certain liabilities, including liabilities under the Securities Act.

INVESTING IN OUR SECURITIES INVOLVES RISKS. SEE THE “RISK FACTORS” ON PAGE S-8 OF THIS PROSPECTUS SUPPLEMENT AND IN THE DOCUMENTS INCORPORATED BY REFERENCE IN THIS PROSPECTUS SUPPLEMENT CONCERNING FACTORS YOU SHOULD CONSIDER BEFORE INVESTING IN OUR COMMON STOCK.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed on the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

Piper Sandler

The date of this prospectus supplement is     , 2023.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS SUPPLEMENT

This prospectus supplement is part of a shelf registration statement on Form S-3 that we filed with the Securities and Exchange Commission, or the SEC. Under the shelf registration process, we may offer shares of our Common Stock having an aggregate offering price of up to $100,000,000. Under this prospectus supplement, we may from time to time sell shares of our Common Stock having an aggregate offering price of up to $10,000,000 at prices and on terms to be determined by market conditions at the time of the offering. The $10,000,000 of shares of our Common Stock that may be sold under this prospectus supplement are included in the $100,000,000 of shares of Common Stock that may be sold under the registration statement.

This prospectus supplement and the accompanying prospectus relate to the offering of our Common Stock. Before buying any of the Common Stock that we are offering, we urge you to carefully read this prospectus supplement and the accompanying prospectus, together with the information incorporated by reference as described under the heading “Incorporation of Certain Information by Reference” in this prospectus supplement. These documents contain important information that you should consider when making your investment decision.

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of the Common Stock we are offering and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference herein or therein. The second part, the accompanying prospectus, provides more general information. Generally, when we refer to this prospectus, we are referring to both this prospectus supplement and the accompanying prospectus. To the extent there is a conflict between the information contained in this prospectus supplement and the information contained in the accompanying prospectus or any document incorporated by reference herein or therein filed prior to the date of this prospectus supplement, you should rely on the information in this prospectus supplement; provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in the accompanying prospectus—the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein or in the accompanying prospectus were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreement, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

You should rely only on the information contained in, or incorporated by reference in this prospectus supplement, the accompanying prospectus and in any free writing prospectus that we may authorize for use in connection with this offering. We have not, and Piper Sandler has not, authorized anyone to provide any information other than that contained or incorporated by reference in this prospectus supplement, in the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We and Piper Sandler take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We are not, and Piper Sandler is not, making an offer to sell, or soliciting an offer to purchase, the securities offered by this prospectus supplement and the accompanying prospectus in any jurisdiction to or from any person to whom or from whom it is unlawful to make such offer or solicitation of an offer in such jurisdiction. The information contained in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein or therein, and in any free writing prospectus prepared by or on behalf of us that we may authorize for use in connection with this filing is accurate only as of the date of those respective documents. Our business, financial condition, results of operations and prospectus may have changed since those dates. It is important for you to read and consider all information

S-1

contained in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and therein, and any free writing prospectus prepared by or on behalf of us that we may authorize for use in connection with this offering, in their entirety, before making an investment decision. You should also read and consider the information in the documents to which we have referred you in the sections entitled “Where You Can Find More Information” and “Incorporation of Certain Information by Reference” in this prospectus supplement and in the accompanying prospectus.

We are offering to sell, and seeking offers to buy, shares of our Common Stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the Common Stock in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the Common Stock and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless otherwise stated, all references in this prospectus supplement and the accompanying prospectus to “we,” “us,” “our,” “Casa,” “the Company” and similar designations refer, collectively, to Casa Systems, Inc., a Delaware corporation, and its consolidated subsidiaries.

S-2

CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the information incorporated by reference in this prospectus include “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Exchange Act. All statements other than statements of historical fact contained in this prospectus, including statements regarding our future results of operations and financial position, business strategy and plans and objectives of management for future operations, are forward-looking statements. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements.

In some cases, you can identify forward-looking statements by terms such as “may,” “will,” “should,” “expects,” “intends,” “plans,” “anticipates,” “could,” “intends,” “target,” “projects,” “seeks,” “contemplates,” “believes,” “estimates,” “endeavor,” “predicts,” “potential” or “continue” or the negative of these terms or other similar expressions. The forward-looking statements in this prospectus are only predictions. We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our business, financial condition and results of operations. Accordingly, these statements involve estimates, assumptions, risks and uncertainties which could cause actual results to differ materially from those expressed in them. These forward-looking statements speak only as of the date of this prospectus and are subject to a number of risks, uncertainties and assumptions described in the “Risk Factors” section and elsewhere in this prospectus and such forward-looking statements are qualified in their entirety by reference to the factors discussed throughout this prospectus, and in particular those factors referenced in the section “Risk Factors.” Because forward-looking statements are inherently subject to risks and uncertainties, some of which cannot be predicted or quantified, you should not rely on these forward-looking statements as predictions of future events. The events and circumstances reflected in our forward-looking statements may not be achieved or occur and actual results could differ materially from those projected in the forward-looking statements.

This prospectus contains forward-looking statements that are based on our management’s belief and assumptions and on information currently available to our management. These statements relate to future events or our future financial performance, and involve known and unknown risks, uncertainties and other factors that may cause our actual results, levels of activity, performance or achievements to be materially different from any future results, levels of activity, performance or achievements expressed or implied by these forward-looking statements. Forward-looking statements include, but are not limited to, statements about:

S-3

This prospectus and the documents incorporated by reference also contain estimates, projections and other information concerning our industry, our business and the markets for our solutions. Information that is based on estimates, forecasts, projections, market research or similar methodologies is inherently subject to uncertainties and actual events or circumstances may differ materially from events and circumstances reflected in this information. Unless otherwise expressly stated, we obtained this industry, business, market and other data from reports, research surveys, studies and similar data prepared by market research firms and other third parties, industry and general publications, government data and similar sources.

You should read this prospectus and the documents that we incorporate by reference in this prospectus completely and with the understanding that our actual future results may be materially different from what we expect. The forward-looking statements in this prospectus and the documents we incorporate by reference herein represent our views as of their respective dates. We anticipate that subsequent events and developments will cause our views to change. However, while we may elect to update these forward-looking statements at some point in the future, we have no current intention of doing so except to the extent required by applicable law. You should, therefore, not rely on these forward-looking statements as representing our views as of any date subsequent to the date of this prospectus

S-4

S-5

S-6

S-7

RISK FACTORS

Investing in our Common Stock involves a high degree of risk. Before you decide to invest in our Common Stock, you should carefully consider the risks and uncertainties described below and in Part II, Item 1A “Risk Factors” of our most recent quarterly report on Form 10-Q filed with the SEC on August 8, 2023, together with all other information contained in this prospectus supplement, the accompanying prospectus and in our filings with the SEC that we have incorporated by reference in this prospectus supplement and the accompanying prospectus. If any of the following risks actually occurs, our business, financial condition or results of operations could be adversely affected, which, in turn, could have a negative impact on the price of our Common Stock. In these circumstances, the market price of our Common Stock could decline, and you may lose all or part of your investment.

Risks Related To Our Common Stock and This Offering

If you purchase shares of Common Stock in this offering, you may suffer immediate and substantial dilution of your investment.

The price per share of our Common Stock in this offering may exceed the net tangible book value per share of our Common Stock outstanding prior to this offering. Therefore, if you purchase shares of our Common Stock in this offering, you may pay a price per share that substantially exceeds our net tangible book value per share after this offering. To the extent shares are issued under outstanding options at exercise prices lower than the price of our Common Stock in this offering, you will incur further dilution. Assuming that an aggregate of 9,433,962 shares of our Common Stock are sold at a price of $1.06 per share, the last reported sale price of our Common Stock on The Nasdaq Global Select Market on August 4, 2023, for aggregate proceeds to us of $10.0 million after deducting commissions and estimated aggregate offering expenses payable by us, you will experience immediate dilution of $1.95 per share, representing the difference between our as adjusted net tangible book value per share as of June 30, 2023 after giving effect to this offering and the assumed offering price. See the section entitled “Dilution” below for a more detailed illustration of the dilution you would incur if you participate in this offering.

We have broad discretion in the use of the net proceeds from this offering and may use them in ways that you and other stockholders may not approve.

We currently expect to use the net proceeds of this offering for working capital and other general corporate purposes. Our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations or enhance the value of our Common Stock. The failure by our management to apply these funds effectively could result in financial losses that could have a material adverse effect on our business, cause the price of our Common Stock to decline and delay the development of our product candidates. Pending their use, we may invest the net proceeds from this offering in a manner that does not produce income or that loses value. See “Use of Proceeds.”

You may experience future dilution as a result of future equity offerings.

To raise additional capital, we may in the future offer additional shares of our Common Stock or other securities convertible into or exchangeable for our Common Stock at prices that may not be the same as the price per share in this offering. We may sell shares or other securities in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our Common Stock, or securities convertible or exchangeable into Common Stock, in future transactions may be higher or lower than the price per share paid by investors in this offering.

S-8

We might not be able to utilize a significant portion of our net operating loss carryforwards and research and development tax credit carryforwards.

As of December 31, 2022, we had approximately $0.769 million of federal net operating losses, or NOLs that can be carried forward indefinitely. As of December 31, 2022, we had approximately $15.449 million of state NOLs. Of the state NOLs, some are indefinite life, but most are definite life with various expiration dates beginning in 2031. As of December 31, 2022, we had federal and state research and development tax credit carryforwards of $0.366 million and $12.830 million, respectively, which begin to expire in 2038 and 2031, respectively. Under current law, federal NOLs generated in taxable years ending after December 31, 2017, may be carried forward indefinitely, but the deductibility of such federal NOLs may be limited to 80% of our taxable income annually for tax years beginning after December 31, 2020. NOLs generated prior to December 31, 2017, however, have a 20-year carryforward period, but are not subject to the 80% limitation.

Under U.S. federal income tax law, a corporation’s ability to utilize its NOLs to offset future taxable income may be significantly limited if it experiences an “ownership change” as defined in Section 382 of the Internal Revenue Code, as amended. In general, an ownership change will occur if there is a cumulative change in a corporation’s ownership by “5 percent shareholders” that exceeds 50 percentage points over a rolling three-year period. A corporation that experiences an ownership change will generally be subject to an annual limitation on the use of its pre-ownership change NOLs equal to the value of the corporation immediately before the ownership change, multiplied by the long-term tax-exempt rate (subject to certain adjustments). Furthermore, our ability to utilize NOLs of companies that we have acquired or may acquire in the future may be subject to similar limitations. We may experience ownership changes in the future as a result of subsequent shifts in our stock ownership, including this offering, some of which may be outside of our control. There is also a risk that due to regulatory changes, such as suspensions on the use of NOLs by federal or state taxing authorities or other unforeseen reasons, our existing NOLs could expire or otherwise be unavailable to reduce future income tax liabilities. For these reasons, we may not be able to utilize a material portion of the NOLs reflected on our balance sheet, even if we attain profitability, which could potentially result in increased future tax liability to us and could adversely affect our operating results and financial condition.

Changes in tax law could adversely affect our business and financial condition.

The rules dealing with U.S. federal, state, and local income taxation are constantly under review by persons involved in the legislative process and by the U.S. Internal Revenue Service and the U.S. Treasury Department. Changes to tax laws (which changes may have retroactive application) could adversely affect us or holders of our Common Stock. In recent years, many such changes have been made and changes are likely to continue to occur in the future. Future changes in tax laws could have a material adverse effect on our business, cash flow, financial condition or results of operations. We urge investors to consult with their legal and tax advisers regarding the implications of changes in tax laws on an investment in our Common Stock.

The actual number of shares we will issue under the sale agreement, at any one time or in total, as well as the gross proceeds resulting from those sales, is uncertain.

Subject to certain limitations in the sale agreement and compliance with applicable law, we have the discretion to deliver a placement notice to Piper Sandler at any time throughout the term of the sale agreement. The number of shares that are sold by Piper Sandler after delivering a placement notice will fluctuate based on the market price of our Common Stock during the sales period and limits we set with Piper Sandler. Because the price per share of each share sold will fluctuate based on the market price of our Common Stock during the sales period, it is not possible at this stage to predict the number of shares, if any, that will be ultimately issued.

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The Common Stock offered hereby will be sold in “at the market offerings,” and investors who buy shares at different times will likely pay different prices.

Investors who purchase shares in this offering at different times will likely pay different prices, and so may experience different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices and numbers of shares sold, and there is no minimum or maximum sales price. Investors may experience a decline in the value of their shares as a result of share sales made at prices lower than the prices they paid.

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USE OF PROCEEDS

We may issue and sell shares of our Common Stock having aggregate sales proceeds of up to $10.0 million from time to time under this prospectus supplement. The amount of the net proceeds to us from this offering will depend upon the number of shares of our Common Stock sold and the price at which they are sold. There can be no assurance that we will be able to sell any shares under or fully utilize the sale agreement with Piper Sandler.

We currently expect to use the net proceeds from the offering for working capital needs and other general corporate purposes.

The expected use of net proceeds from this offering represents our intentions based upon our current plans and business conditions, which could change in the future as our plans and business conditions evolve. We have not determined the amounts we plan to spend on any of the areas listed above or the timing of these expenditures. The amounts and timing of our actual expenditures may vary significantly depending on numerous factors, including the progress of our development, the status of and results from clinical trials and collaborations that we may enter into with third parties for our product candidates and any unforeseen cash needs, including the factors described under “Risk Factors” in this prospectus supplements and in the documents incorporated by reference herein. As a result, our management will retain broad discretion over the allocation of the net proceeds from this offering. We may find it necessary or advisable to use the net proceeds from this offering for other purposes, and we will have broad discretion in the application of net proceeds.

We do not expect that the net proceeds from this offering, together with our existing cash, cash equivalents and investments, will be sufficient to enable us to fund the completion of development of any of our product candidates.

Pending our use of the net proceeds from this offering, we intend to invest the net proceeds in a variety of capital preservation investments, including short-term, investment-grade, interest-bearing instruments and U.S. government securities.

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DILUTION

If you invest in this offering, your ownership interest will be diluted immediately to the extent of the difference between the public offering price per share and the as-adjusted net tangible book value per share of our Common Stock after giving effect to this offering.

As of June 30, 2023, we had net tangible book value of approximately $(104.1) million, or $(1.07) per share of our Common Stock, based upon 97,012,816 shares of our Common Stock outstanding as of that date. Historical net tangible book value per share is equal to our total tangible assets, less total liabilities, divided by the number of outstanding shares of our Common Stock. Dilution in net tangible book value per share represents the difference between the amount per share paid by purchasers of shares of Common Stock in this offering and the net tangible book value per share of our Common Stock immediately after this offering.

After giving effect to the sale of our Common Stock pursuant to this prospectus supplement and accompanying prospectus in the aggregate amount of $10.0 million at an assumed offering price of $1.06 per share, the last reported sale price of our Common Stock on The Nasdaq Global Select Market on August 4, 2023, and after deducting commissions and estimated aggregate offering expenses payable by us, our net tangible book value as of June 30, 2023 would have been $(94.8) million, or $(0.89) per share of Common Stock. This represents an immediate increase in the net tangible book value of $0.18 per share to our existing stockholders and an immediate dilution in net tangible book value of $1.95 per share to new investors. The following table illustrates this per share dilution:

The table above assumes for illustrative purposes that an aggregate of 9,433,962 shares of our Common Stock are sold pursuant to this prospectus supplement and the accompanying prospectus at a price of $1.06 per share, the last reported sale price of our Common Stock on The Nasdaq Global Select Market on August 4, 2023, for aggregate gross proceeds of $10.0 million. The shares sold in this offering, if any, will be sold from time to time at various prices. An increase of $1.00 per share in the price at which the shares are sold from the assumed offering price to $2.06 per share, assuming all of our Common Stock in the aggregate amount of $10.0 million is sold at that price, would result in an adjusted net tangible book value per share after the offering of $(0.93) per share and would increase the dilution in net tangible book value per share to new investors in this offering to $2.99 per share, after deducting commissions and estimated aggregate offering expenses payable by us. A decrease of $1.00 per share in the price at which the shares are sold from the assumed offering price to $0.06 per share, assuming all of our Common Stock in the aggregate amount of $10.0 million is sold at that price, would result in an adjusted net tangible book value per share after the offering of $(0.36) per share and would decrease the dilution in net tangible book value per share to new investors in this offering to $0.42 per share, after deducting commissions and estimated aggregate offering expenses payable by us. This information is supplied for illustrative purposes only.

The above discussion and table are based on 97,012,816 shares of our Common Stock outstanding as of June 30, 2023 and excludes the following:

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DIVIDEND POLICY

We have previously declared or paid special cash dividends on our capital stock on five occasions prior to our initial public offering. We currently plan to retain all of our future earnings, if any, to finance the operation, development and growth of our business. We do not expect to pay any cash dividends on our Common Stock in the foreseeable future. Payment of future dividends, if any, will be at the discretion of our board of directors and will depend on our financial condition, results of operations, capital requirements, restrictions contained in current or future financing instruments, provisions of applicable law and other factors the board deems relevant. Our ability to pay dividends on our Common Stock may be restricted by the terms of any of our future indebtedness. Our credit facility contains covenants that limit our ability to pay dividends on our capital stock.

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MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS

The following discussion is a summary of certain material U.S. federal income tax considerations applicable to non-U.S. holders (as defined below) with respect to their ownership and disposition of shares of our Common Stock issued pursuant to this offering. For purposes of this discussion, a non-U.S. holder means a beneficial owner of our Common Stock that is for U.S. federal income tax purposes:

This discussion does not address the tax treatment of partnerships or other entities that are pass-through entities for U.S. federal income tax purposes or persons that hold their Common Stock through partnerships or other pass-through entities. A partner in a partnership or other pass-through entity that will hold our Common Stock should consult his, her or its tax advisor regarding the tax consequences of acquiring, holding and disposing of our Common Stock through a partnership or other pass-through entity, as applicable.

This discussion is based on current provisions of the Code, existing and proposed U.S. Treasury Regulations promulgated thereunder, current administrative rulings and judicial decisions, all as in effect as of the date of this prospectus and, all of which are subject to change or to differing interpretation, possibly with retroactive effect. Any such change or differing interpretation could alter the tax consequences to non-U.S. holders described in this prospectus. There can be no assurance that the Internal Revenue Service, which we refer to as the IRS, will not challenge one or more of the tax consequences described herein. We have not sought and will not seek any rulings from the IRS regarding the matters discussed below. We assume in this discussion that a non-U.S. holder holds shares of our Common Stock as a capital asset within the meaning of Section 1221 of the Code, generally property held for investment.

This discussion does not address all aspects of U.S. federal income taxation that may be relevant to a particular non-U.S. holder in light of that non-U.S. holder’s individual circumstances nor does it address any U.S. state, local or non-U.S. taxes, the alternative minimum tax, the Medicare tax on net investment income, the rules regarding qualified small business stock within the meaning of Section 1202 of the Code, or any other aspect of any U.S. federal tax other than the income tax. This discussion also does not consider any specific facts or circumstances that may apply to a non-U.S. holder and does not address the special tax rules applicable to particular non-U.S. holders, such as:

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This discussion is for general information only and is not tax advice. Accordingly, all prospective non-U.S. holders of our Common Stock should consult their tax advisors with respect to the U.S. federal, state, local and non-U.S. tax consequences of the purchase, ownership and disposition of our Common Stock.

Distributions on Our Common Stock

As described in the “Dividend Policy” section above, we do not anticipate paying any cash dividends on our capital stock in the foreseeable future. Distributions, if any, on our Common Stock will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. If a distribution exceeds our current and accumulated earnings and profits, the excess will be treated as a tax-free return of the non-U.S. holder’s investment, up to such holder’s tax basis in the Common Stock. Any remaining excess will be treated as capital gain, subject to the tax treatment described below in “—Gain on Sale or Other Taxable Disposition of Our Common Stock.” Any such distributions will also be subject to the discussions below under the sections titled “—Backup Withholding and Information Reporting” and “—Withholding and Information Reporting Requirements—FATCA.”

Subject to the discussion in the following two paragraphs in this section, dividends paid to a non-U.S. holder generally will be subject to withholding of U.S. federal income tax at a 30% rate or such lower rate as may be specified by an applicable income tax treaty between the United States and such holder’s country of residence. If we or the applicable withholding agent are unable to determine, at a time reasonably close to the date of payment of a distribution on our Common Stock, what portion, if any, of the distribution will constitute a dividend, then we or the applicable withholding agent may withhold U.S. federal income tax on the basis of assuming that the full amount of the distribution will be a dividend. If we or another withholding agent apply over-withholding, a non-U.S. holder may be entitled to a refund or credit of any excess tax withheld by timely filing an appropriate claim with the IRS.

Dividends that are treated as effectively connected with a trade or business conducted by a non-U.S. holder within the United States and, if an applicable income tax treaty so provides, that are attributable to a permanent establishment or a fixed base maintained by the non-U.S. holder within the United States, are generally exempt from the 30% withholding tax if the non-U.S. holder satisfies applicable certification and disclosure requirements. However, such U.S. effectively connected income, net of specified deductions and credits, is taxed at the same graduated U.S. federal income tax rates applicable to United States persons (as defined in the Code). Any U.S. effectively connected income received by a non-U.S. holder that is a corporation may also, under certain circumstances, be subject to an additional “branch profits tax” at a 30% rate or such lower rate as may be specified by an applicable income tax treaty between the United States and such holder’s country of residence.

A non-U.S. holder of our Common Stock who claims the benefit of an applicable income tax treaty between the United States and such holder’s country of residence generally will be required to provide a properly executed IRS Form W-8BEN or W-8BEN-E (or applicable successor form) to the applicable withholding agent and satisfy

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applicable certification and other requirements. Non-U.S. holders are urged to consult their own tax advisors regarding their entitlement to benefits under a relevant income tax treaty. A non-U.S. holder that is eligible for a reduced rate of U.S. withholding tax under an income tax treaty may obtain a refund or credit of any excess amounts withheld by timely filing a U.S. tax return with the IRS.

Gain on Sale or Other Taxable Disposition of Our Common Stock

Subject to the discussions below under “—Backup Withholding and Information Reporting” and “—Withholding and Information Reporting Requirements—FATCA,” a non-U.S. holder generally will not be subject to any U.S. federal income or withholding tax on any gain realized upon such holder’s sale or other taxable disposition of shares of our Common Stock unless:

Backup Withholding and Information Reporting

We (or the applicable paying agent) must report annually to the IRS and to each non-U.S. holder the gross amount of the distributions on our Common Stock paid to such holder and the tax withheld, if any, with respect to such distributions. Non-U.S. holders may have to comply with specific certification procedures to establish that the holder is not a United States person (as defined in the Code) in order to avoid backup withholding at the applicable rate with respect to dividends on our Common Stock. Generally, a non-U.S. holder will comply with such procedures if it provides a properly executed IRS Form W-8BEN or W-8BEN-E (or other applicable Form

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W-8) or otherwise meets documentary evidence requirements for establishing that it is a non-U.S. holder, or otherwise establishes an exemption. Dividends paid to non-U.S. holders subject to withholding of U.S. federal income tax, as described above in “—Distributions on Our Common Stock,” generally will be exempt from U.S. backup withholding. However, information returns are required to be filed with the IRS in connection with any dividends on our Common Stock paid to the Non-U.S. Holder, regardless of whether any tax was actually withheld

Information reporting and backup withholding will generally apply to the proceeds of a disposition of our Common Stock by a non-U.S. holder effected by or through the U.S. office of any broker, U.S. or foreign, unless the holder certifies its status as a non-U.S. holder and satisfies certain other requirements, or otherwise establishes an exemption. Generally, information reporting and backup withholding will not apply to a payment of disposition proceeds to a non-U.S. holder where the transaction is effected outside the United States through a non-U.S. office of a broker. However, for information reporting purposes, dispositions effected through a non-U.S. office of a broker with substantial U.S. ownership or operations generally will be treated in a manner similar to dispositions effected through a U.S. office of a broker.

Non-U.S. holders should consult their tax advisors regarding the application of the information reporting and backup withholding rules to them. Copies of information returns may be made available to the tax authorities of the country in which the non-U.S. holder resides or is incorporated under the provisions of a specific treaty or agreement. Any documentation provided to an applicable withholding agent may need to be updated in certain circumstances. Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules from a payment to a non-U.S. holder can be refunded or credited against the non-U.S. holder’s U.S. federal income tax liability, if any, provided that an appropriate claim is filed with the IRS in a timely manner.

Withholding and Information Reporting Requirements—FATCA

Provisions of the Code commonly referred to as the Foreign Account Tax Compliance Act, or FATCA, generally impose a U.S. federal withholding tax at a rate of 30% on payments of dividends on our Common Stock paid to a foreign entity unless (i) if the foreign entity is a “foreign financial institution,” such foreign entity undertakes certain due diligence, reporting, withholding, and certification obligations, (ii) if the foreign entity is not a “foreign financial institution,” such foreign entity identifies certain of its U.S. investors, if any, or (iii) the foreign entity is otherwise exempt under FATCA. Such withholding may also apply to gross proceeds from the sale or other disposition of our Common Stock, although under proposed U.S. Treasury Regulations, no withholding would apply to such gross proceeds. The preamble to the proposed regulations specifies that taxpayers (including withholding agents) are permitted to rely on the proposed regulations pending finalization. If withholding under FATCA is required on any payment related to our Common Stock, under certain circumstances, a non-U.S. holder may be eligible for refunds or credits of this withholding tax. An intergovernmental agreement between the United States and an applicable foreign country may modify the requirements described in this section. Non-U.S. holders should consult their tax advisors regarding the possible implications of FATCA on their investment in our Common Stock and the entities through which they hold our Common Stock, including, without limitation, the process and deadlines for meeting the applicable requirements to prevent the imposition of the 30% withholding tax under FATCA

The preceding discussion of material U.S. federal tax considerations is for general information only. It is not tax advice. Prospective investors should consult their tax advisors regarding the particular U.S. federal, state, local and non-U.S. tax consequences of purchasing, holding and disposing of our Common Stock, including the consequences of any proposed changes in applicable laws.

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PLAN OF DISTRIBUTION

We have entered into an equity distribution agreement, or the sale agreement, with Piper Sandler & Co., or Piper Sandler, under which we may issue and sell shares of our Common Stock from time to time up to an aggregate offering price of $10,000,000 through Piper Sandler acting as our sales agent. The summary of the material provisions of the sale agreement does not purport to be a complete statement of its terms and conditions. The sale agreement has been filed with the SEC as an exhibit to a report filed under the Exchange Act and incorporated by reference into this prospectus supplement .

Upon delivery of a placement notice, and subject to our instructions in that notice, and the terms and conditions of the sale agreement generally, Piper Sandler may sell our Common Stock by any method permitted by law deemed to be an “at the market offering” as defined by Rule 415(a)(4) promulgated under the Securities Act. The Common Stock will be distributed at the market prices prevailing on The Nasdaq Global Select Market at the time of the sale of such Common Stock.

We will pay Piper Sandler in cash, upon each sale of our Common Stock pursuant to the sale agreement, a commission in an amount equal to 3.0% of the aggregate gross sales price from each sale of our Common Stock. Because there is no minimum offering amount required as a condition to this offering, the actual total public offering amount, commissions and proceeds to us, if any, are not determinable at this time. We have agreed to reimburse a portion of Piper Sandler expenses, including legal fees, in connection with this offering up to a maximum of $125,000 in connection with the establishment of this at-the-market offering program. In accordance with FINRA Rule 5110 these reimbursed fees and expenses are deemed sales compensation to Piper Sandler in connection with this offering. We estimate that the total expenses for the offering, excluding compensation and expense reimbursement payable to Piper Sandler under the terms of the sale agreement, will be approximately $0.4 million.

Settlement for sales of Common Stock will occur on the second full business day following the date on which any sales are made, or on some other date that is agreed upon by us and Piper Sandler in connection with a particular transaction, in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement. Sales of our Common Stock as contemplated in this prospectus supplement will be settled through the facilities of The Depository Trust Company or by such other means as we and Piper Sandler may agree upon.

Piper Sandler will act as sales agent on a commercially reasonable efforts basis consistent with its normal trading and sales practices and applicable state and federal laws, rules and regulations and the rules of The Nasdaq Global Select Market. In connection with the sale of Common Stock on our behalf, Piper Sandler will be deemed to be an “underwriter” within the meaning of the Securities Act and the compensation of Piper Sandler will be deemed to be underwriting commissions or discounts. We have agreed to provide indemnification and contribution to Piper Sandler against certain civil liabilities, including liabilities under the Securities Act.

The offering of our Common Stock pursuant to the sale agreement will terminate as permitted therein. We or Piper Sandler may terminate the equity distribution agreement at any time with prior notice.

Piper Sandler and its affiliates may in the future provide various investment banking, commercial banking and other financial services for us and our affiliates, for which services they may in the future receive customary fees.

This prospectus supplement and the accompanying prospectus in electronic format may be made available on a website maintained by Piper Sandler and Piper Sandler may distribute this prospectus supplement and the accompanying prospectus electronically.

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LEGAL MATTERS

The validity of the shares of Common Stock offered hereby will be passed upon for us by Goodwin Procter LLP, Boston, Massachusetts. Piper Sandler is being represented in connection with this offering by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., New York, New York.

EXPERTS

The consolidated financial statements of Casa Systems, Inc. appearing in Casa Systems, Inc.’s Annual Report (Form 10-K) for the year ended December 31, 2022, and the effectiveness of Casa Systems, Inc.’s internal control over financial reporting as of December 31, 2022, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in its reports thereon (which contains an explanatory paragraph describing conditions that raise substantial doubt about the Company’s ability to continue as a going concern as described in Note 1 to the consolidated financial statements), which conclude, among other things, that Casa Systems, Inc. did not maintain effective internal control over financial reporting as of December 31, 2022, based on Internal Control—Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 Framework), because of the effects of the material weaknesses described therein, included therein, and incorporated herein by reference. Such financial statements have been incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION; INCORPORATION BY REFERENCEINFORMATION

Available Information

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at www.sec.gov. Copies of certain information filed by us with the SEC are also available on our website at www.casa-systems.com. Our website is not a part of this prospectus supplement and is not incorporated by reference in this prospectus.prospectus supplement.

This prospectus supplement is part of a registration statement on Form S-3that we filed with the SEC. ThisThe registration statement contains more information than this prospectus omits some information contained insupplement and the accompanying prospectus regarding us and the securities, including certain exhibits and schedules. You can obtain a copy of the registration statement in accordance with SEC rules and regulations. You should review the information and exhibits in the registration statement for further information about us and our consolidated subsidiaries and the securities we are offering. Statements in this prospectus concerning any document we filed as an exhibit to the registration statement or that we otherwise filed withfrom the SEC are not intended to be comprehensive and are qualified by reference to these filings. You should reviewat the complete document to evaluate these statements.address listed above or from the SEC’s internet site.

Incorporation by Reference

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INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to incorporate by reference in this prospectus supplement and the accompanying prospectus much of the information we file with the SEC, which means that we can disclose important information to you by referring you to those publicly available documents. The information that we incorporate by reference in this prospectus supplement and the accompanying prospectus is considered to be part of this prospectus supplement and the accompanying prospectus. Because we are incorporating by reference future filings with the SEC, this prospectus supplement and the accompanying prospectus is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus supplement and the accompanying prospectus. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus supplement, the accompanying prospectus or in any document previously incorporated by reference have been modified or superseded. This prospectus incorporatessupplement and the accompanying prospectus incorporate by reference the documents listed below (File No. 001-36310) and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act (in each case, other than those documents or the portions of those documents not deemed to be filed) between the date of the initial registration statement and the effectiveness of the registration statement and following the effectiveness of the registration statement until the offering of the securities under the registration statement is terminated or completed:

You may request a copy of these filings, at no cost, by writing or telephoning us at the following address or telephonephone number:

100 Old River Road

Andover, Massachusetts 01810

(978) 688-6706

In accordance with Rule 412 of the Securities Act, any statement contained in a document incorporated by reference herein shall be deemed modified or superseded to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement.

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CASA SYSTEMS, INC.

Up to $10,000,000

Common Stock

PROSPECTUS SUPPLEMENT

PIPER SANDLER

                , 2023

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and DistributionDistribution.

The following is an estimate oftable sets forth the various expenses (all of which are to be paid by the registrant) that we may incurincurred in connection with the sale and distribution of the securities being registered hereby.registered.

Item 15. Indemnification of Directors and Officers.

Section 145 of the DGCL authorizes a corporation to indemnify its directors and officers against liabilities arising out of actions, suits and proceedings to which they are made or threatened to be made a party by reason of the fact that they have served or are currently serving as a director or officer to a corporation. The indemnity may cover expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by the director or officer in connection with any such action, suit or proceeding. Section 145 permits corporations to pay expenses (including attorneys’ fees) incurred by directors and officers in advance of the final disposition of such action, suit or proceeding. In addition, Section 145 provides that a corporation has the power to purchase and maintain insurance on behalf of its directors and officers against any liability asserted against them and incurred by them in their capacity as a director or officer, or arising out of their status as such, whether or not the corporation would have the power to indemnify the director or officer against such liability under Section 145.

We have adopted provisions in our amended and restated certificate of incorporation and amended and restated by-laws that limit or eliminate the personal liability of our directors to the fullest extent permitted by the DGCL, as it now exists or may in the future be amended. Consequently, a director will not be personally liable to us or our stockholders for monetary damages or breach of fiduciary duty as a director, except for liability for:

These limitations of liability do not alter director liability under the federal securities laws and do not affect the availability of equitable remedies such as an injunction or rescission.

In addition, our amended and restated by-laws provide that:

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We have entered into indemnification agreements with each of our directors and with certain of our executive officers. These agreements provide that we will indemnify each of our directors, certain of our executive officers and, at times, their affiliates to the fullest extent permitted by Delaware law. We will advance expenses, including attorneys’ fees (but excluding judgments, fines and settlement amounts), to each indemnified director, executive officer or affiliate in connection with any proceeding in which indemnification is available and we will indemnify our directors and officers for any action or proceeding arising out of that person’s services as a director or officer brought on behalf of us and/or in furtherance of our rights. Additionally, each of our directors may have certain rights to indemnification, advancement of expenses and/or insurance provided by their affiliates, which indemnification relates to and might apply to the same proceedings arising out of such director’s services as a director referenced herein. Nonetheless, we have agreed in the indemnification agreements that our obligations to those same directors are primary and any obligation of the affiliates of those directors to advance expenses or to provide indemnification for the expenses or liabilities incurred by those directors are secondary.

We also maintain general liability insurance which covers certain liabilities of our directors and officers arising out of claims based on acts or omissions in their capacities as directors or officers, including liabilities under the Securities Act.

Item 16. Exhibits.

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EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act, we certify that we have reasonable grounds to believe that we meet all of the requirements for filing on Form S-3 and have duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the town of Andover, Commonwealth of Massachusetts, on this 21st8th day of July,August, 2023.

POWER OF ATTORNEY AND SIGNATURES

We, the undersigned officers and directors of Casa Systems, Inc., hereby severally constitute and appoint each of Michael Glickman and Edward Durkin, and each of them singly (with full power to each of them to act alone), as our true and lawful attorneyattorneys with full power to any of them, and each of them acting singly, to sign for us and in our names in the capacities indicated below the Registration Statement on Form S-3 filed herewith and any and all amendments (including post-effective amendments) to said Registration Statement, and any registration statement filed pursuant to Rule 462 under the Securities Act of 1933, as amended, in connection with said Registration Statement, and to file or cause to be filed the same, with all exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, and generally to do all such things in our name and on our behalf in our capacities as officers and directors to enable Casa Systems, Inc. to comply with the provisions of the Securities Act of 1933, as amended, and all requirements of the Securities and Exchange Commission, hereby ratifying and confirming all that said attorneys, and each of them, or their substitute or substitutes, shall do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

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