As filed with the Securities and Exchange Commission on December 20, 2023January 11, 2024

Registration No. 333-275479

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Amendment No. 12

to

FORM S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

EASTERN BANKSHARES, INC.

(Exact name of registrant as specified in its charter)

265 Franklin Street

Boston, Massachusetts 02110

(800) 327-8376

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Kathleen C. Henry, Esq.

c/o Eastern Bankshares, Inc.

265 Franklin Street

Boston, Massachusetts 02110

(Name, address, including zip code, and telephone number, including area code, of agent for service)

With copies to:

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement is declared effective and upon completion of the merger described therein.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, 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, 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(d) 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.  ☐

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

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.  ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

THE REGISTRANT 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 THAT SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE SECURITIES ACT OR UNTIL THIS REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES EXCHANGE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A), MAY DETERMINE.

The information in this joint proxy statement/prospectus is not complete and may be changed. This joint proxy statement/prospectus is not an offer to sell these securities nor should it be considered a solicitation of an offer to buy these securities in any jurisdiction in which such offer, solicitation, or sale is not permitted.

PRELIMINARY—SUBJECT TO COMPLETION—DATED DECEMBER 20, 2023JANUARY 11, 2024

JOINT PROXY STATEMENT/PROSPECTUS

Dear Eastern and Cambridge Shareholders:

On behalf of the Boards of Directors of Eastern Bankshares, Inc. (“Eastern”) and Cambridge Bancorp (“Cambridge”), we are pleased to enclose the accompanying joint proxy statement/prospectus relating to the acquisition of Cambridge by Eastern. We are requesting that you take certain actions as a holder of Eastern common stock (an “Eastern shareholder”) or as a holder of Cambridge common stock (a “Cambridge shareholder”).

On September 19, 2023, Eastern, Eastern Bank, a Massachusetts-chartered trust company and wholly-owned subsidiary of Eastern (“Eastern Bank”), Citadel MS 2023, Inc., a direct, wholly-owned subsidiary of Eastern (“Merger Sub”), Cambridge and Cambridge Trust Company, a Massachusetts-chartered trust company and wholly-owned subsidiary of Cambridge (“Cambridge Trust”), entered into an Agreement and Plan of Merger (the “merger agreement”), pursuant to which Eastern will, upon the terms and subject to the conditions set forth in the merger agreement, acquire Cambridge in an all-stock transaction. The merger (as defined below) will create a combined franchise with approximately $27 billion in total assets, the largest bank-owned independent investment advisor in Massachusetts, and further solidify Eastern as the largest Boston-based community bank by deposits.

Under the merger agreement, Merger Sub will merge with and into Cambridge, with Cambridge as the surviving entity (the “merger”), and as soon as reasonably practicable following the merger, Cambridge will merge with and into Eastern, with Eastern as the surviving entity (the “holdco merger”). Following the holdco merger, at a time to be determined by Eastern, Cambridge Trust will merge with and into Eastern Bank, with Eastern Bank as the surviving bank (the “bank merger” and together with the merger and the holdco merger, the “mergers”).

If the merger is completed, Cambridge shareholders will receive 4.956 shares of Eastern common stock for each share of Cambridge common stock they own on the effective date of the merger. Cambridge shareholders will also receive cash in lieu of any fractional shares of Eastern common stock they would have otherwise received in the merger. The precise value of the consideration that Cambridge shareholders will receive will not be known at the time that Cambridge shareholders vote on the Cambridge Merger Proposal (as defined below). On September 18, 2023, which was the last trading day preceding the public announcement of the proposed merger, the closing price of Eastern common stock was $13.41 per share, which after giving effect to the exchange ratio has an implied value of $66.46 per Cambridge share. On             , 2024, which was the most recent practicable trading day before the printing of this joint proxy statement/prospectus, the closing price of Eastern common stock was $         per share, which after giving effect to the exchange ratio, has an implied value of approximately $         per Cambridge share.

Eastern shareholders will continue to own their existing shares of Eastern common stock. The value of the Eastern common stock at the time of completion of the merger could be greater than, less than or the same as the value of Eastern common stock on the date of the accompanying joint proxy statement/prospectus. We urge you to obtain current market quotations of Eastern common stock (trading symbol “EBC”) and Cambridge common stock (trading symbol “CATC”), both of which are traded on the Nasdaq Global Select Market.

We expect the merger and the holdco merger, taken together, will qualify as a reorganization for federal income tax purposes. Accordingly, Cambridge shareholders generally will not recognize any gain or loss for federal income tax purposes on the exchange of shares of Cambridge common stock for Eastern common stock in the merger, except with respect to any cash received by such holders in lieu of fractional shares of Eastern common stock.

Based on the number of shares of Cambridge common stock outstanding or reserved for issuance as of September 18, 2023, Eastern expects to issue approximately 39.4 million shares of Eastern common stock to Cambridge shareholders in the aggregate in the merger. We estimate that, immediately following the completion of the merger, former Cambridge shareholders will own approximately 18% and existing Eastern shareholders will own approximately 82% of the common stock of Eastern.

Eastern and Cambridge will each hold a special meeting of their respective shareholders in connection with the merger. At the respective special meetings, in addition to other business, Eastern will ask its shareholders to approve the issuance of its common stock to holders of Cambridge common stock pursuant to the merger agreement (the “Eastern Share Issuance Proposal”), and Cambridge will ask its shareholders to approve the merger agreement (the “Cambridge Merger Proposal”). Information about these meetings and the mergers is contained in this document. We urge you to read this document carefully and in its entirety.

The special meeting of Cambridge shareholders will be held virtually via webcast at                      on          at          , Eastern Time. The special meeting of Eastern shareholders will be held virtually via webcast at                      , on                  at         , Eastern Time.

Each of the Eastern board of directors and the Cambridge board of directors unanimously recommends that holders of Eastern common stock and Cambridge common stock, respectively, vote “FOR” each of the proposals to be considered at the respective meetings. We strongly support this combination of our companies and join our boards in their recommendations.

This joint proxy statement/prospectus provides you with detailed information about the merger agreement and the merger. It also contains or references information about Eastern and Cambridge and certain related matters. You are encouraged to read this joint proxy statement/prospectus carefully. In particular, you should read the “Risk Factors” section beginning on page 28 for a discussion of the risks you should consider in evaluating the proposed merger and how it will affect you. You can also obtain information about Eastern and Cambridge from documents that have been filed with the Securities and Exchange Commission that are incorporated into this joint proxy statement/prospectus by reference.

Your vote is very important. Approval of the Cambridge Merger Proposal will require the affirmative vote of the holders of two-thirds of the shares of Cambridge common stock outstanding and entitled to vote on the matter. Approval of the Eastern Share Issuance Proposal will require the affirmative vote of a majority of the votes cast by the holders of Eastern common stock at the Eastern special meeting, assuming a quorum is present at the virtual meeting. Whether or not you plan to attend the Cambridge special meeting or Eastern special meeting, as applicable, please take the time to vote by completing and mailing the enclosed proxy card, or through our online voting system accessible at                                     . (If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.) If Cambridge shareholders do not vote virtually or by proxy, it will have the same effect as a vote against the proposal to approve the merger at the Cambridge special meeting.

Neither the Securities and Exchange Commission nor any state securities commission or bank regulatory agency has approved or disapproved of the proposed merger or the securities to be issued under this joint proxy statement/prospectus, or passed upon the adequacy or accuracy of this joint proxy statement/prospectus. Any representation to the contrary is a criminal offense.

The shares of Eastern common stock to be issued in the merger are not savings accounts, deposits or other obligations of any bank or savings association and are not insured by the Federal Deposit Insurance Corporation or by any other federal or state government or governmental agency.

The accompanying joint proxy statement/prospectus is dated                    , and is first being mailed to Eastern and Cambridge shareholders and on or about                    .

Cambridge Bancorp

1336 Massachusetts Avenue

Cambridge, Massachusetts 02138

(617) 876-5500

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD ON

To the Shareholders of Cambridge Bancorp:

A special meeting of the shareholders of Cambridge Bancorp, a Massachusetts corporation (“Cambridge”), will be held virtually via webcast at          on          at         , Eastern Time (the “Cambridge special meeting”), for the purpose of considering and voting on the following matters:

Cambridge does not expect to transact any business other than as set forth above at the Cambridge special meeting or any postponement or adjournment thereof. Please refer to the attached joint proxy statement/prospectus for further information with respect to the business to be transacted at the Cambridge special meeting. The board of directors of Cambridge has fixed the close of business on , 2023Monday, January 8, 2024 as the record date for the determination of Cambridge shareholders entitled to receive notice of, and to vote at, the Cambridge special meeting and any postponements or adjournments of the Cambridge special meeting. Only holders of record of shares of Cambridge common stock at the close of business on the record date are entitled to receive notice of, and to vote at, the Cambridge special meeting.

Approval of the Cambridge Merger Proposal requires the affirmative vote of the holders of two-thirds of the outstanding shares of Cambridge common stock entitled to vote on such proposal. If a Cambridge shareholder does not vote on the Cambridge Merger Proposal, it will have the same effect as a vote by such holder against the proposal. The transactions contemplated by the merger agreement cannot be completed unless Cambridge shareholders approve the Cambridge Merger Proposal.

Assuming a quorum is present, approval of the Cambridge Compensation Proposal requires the affirmative vote of a majority of the votes cast on such proposal.

Assuming a quorum is present, approval of the Cambridge Adjournment Proposal requires the affirmative vote of a majority of the votes cast on such proposal.

To register to attend the Cambridge special meeting virtually via the Internet, visit                          and enter your control number. Cambridge shareholders who register to attend will be provided a link to the virtual meeting on the day of the meeting.

Under the provisions of the Massachusetts Business Corporation Act, as amended, the holders of Cambridge common stock are not entitled to dissenters’ rights of appraisal in connection with the merger.

The Cambridge board of directors has unanimously approved the merger agreement, the merger and the other transactions contemplated by the merger agreement and unanimously recommends that the Cambridge shareholders vote “FOR” the Cambridge Merger Proposal, “FOR” the Cambridge Compensation Proposal and “FOR” the Cambridge Adjournment Proposal.

The affirmative vote of the holders of two-thirds of the shares of Cambridge common stock outstanding and entitled to vote at the Cambridge special meeting is required to approve the Cambridge Merger Proposal. The affirmative vote of a majority of the votes cast, in person or represented by proxy, at the Cambridge special meeting is required to approve the Cambridge Compensation Proposal and the Cambridge Adjournment Proposal. Whether or not you plan to attend the Cambridge special meeting, we urge you to please promptly complete, sign, date and return the accompanying proxy card in the enclosed postage-paid envelope or authorize the individuals named on the accompanying proxy card to vote your shares by calling the toll-free telephone number or by using the internet as described in the instructions included with the accompanying proxy card. If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.

IF YOU HAVE ANY QUESTIONS OR NEED ASSISTANCE VOTING YOUR SHARES, PLEASE CONTACT OUR PROXY SOLICITOR, ALLIANCE ADVISORS, LLC, AT 844-984-3713.

Eastern Bankshares, Inc.

265 Franklin Street

Boston, Massachusetts 02110

(800) 327-8376

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS TO BE HELD ON

To the Shareholders of Eastern Bankshares, Inc.:

A special meeting of the shareholders of Eastern Bankshares, Inc., a Massachusetts corporation (“Eastern”), will be held virtually via webcast at                      on                      at                     , Eastern Time (the “Eastern special meeting”), for the purpose of considering and voting on the following matters:

Eastern does not expect to transact any business other than as set forth above at the Eastern special meeting or any postponement or adjournment thereof. Please refer to the attached joint proxy statement/prospectus for further information with respect to the business to be transacted at the Eastern special meeting. The board of directors of Eastern has fixed the close of business on             , 2023Monday, January 8, 2024 as the record date for the determination of Eastern shareholders entitled to receive notice of, and to vote at, the Eastern special meeting and any postponements or adjournments of the Eastern special meeting. Only holders of record of shares of Eastern common stock at the close of business on the record date are entitled to receive notice of, and to vote at, the Eastern special meeting.

Eastern’s bylaws require that the holders of a majority of the shares of Eastern common stock, issued and outstanding and entitled to vote at the meeting, be present, or represented by proxy at the meeting in order to constitute a quorum for the transaction of business. Accordingly, it is important that your shares be represented at the meeting regardless of the number of shares you may hold. Please ensure that your shares of Eastern common stock are present and voted at the meeting by submitting your instructions by telephone, the Internet, or in writing by completing, signing, dating and returning your proxy card or voting form. You may also vote your shares virtually through the Internet during the meeting.

To register to attend the Eastern special meeting virtually via the Internet, visit                                  and enter your control number. Eastern shareholders who register to attend will be provided a link to the virtual meeting on the day of the meeting.

To vote your shares virtually in advance of or at the Eastern special meeting, visit www.proxypush.com/EBC and enter your control number. Your control number can be found on your proxy card, vote authorization form or the Notice of Internet Availability of Proxy Materials.

Under the provisions of the Massachusetts Business Corporation Act, as amended, the holders of Eastern common stock are not entitled to dissenters’ rights of appraisal in connection with the merger.

The Eastern board of directors unanimously recommends that holders of Eastern common stock vote “FOR” the Eastern Share Issuance Proposal and “FOR” the Eastern Adjournment Proposal.

Your vote is important. Your vote is important regardless of how many shares you own. We cannot complete the transactions contemplated by the merger agreement unless Eastern shareholders approve the Eastern Share Issuance Proposal. The affirmative vote of a majority of the votes cast by Eastern shareholders at the Eastern special meeting is required to approve the Eastern Share Issuance Proposal and the Eastern Adjournment Proposal.

Whether or not you plan to attend the Eastern special meeting, we urge you to please promptly complete, sign, date and return the accompanying proxy card in the enclosed postage-paid envelope or authorize the individuals named on the accompanying proxy card to vote your shares by calling the toll-free telephone number or by using the internet as described in the instructions included with the accompanying proxy card. If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.

REFERENCE TO ADDITIONAL INFORMATION

This joint proxy statement/prospectus incorporates important business and financial information about Cambridge and Eastern from other documents that are not included in, or delivered with, this joint proxy statement/prospectus. This information is available to you without charge upon your written or oral request. We have listed the documents containing this information in the section titled “Incorporation of Certain Documents by Reference” beginning on page 149152 of this joint proxy statement/prospectus. You can obtain copies of these documents incorporated by reference in this document through the Securities and Exchange Commission’s website at http://www.sec.gov or by requesting them in writing or by telephone from Cambridge or Eastern at the following addresses:

For business and financial information about Cambridge, please contact:

Cambridge Investor Relations

1336 Massachusetts Avenue

Cambridge, Massachusetts 02138

Email: InvestorRelations@cambridgetrust.com

Telephone: (617) 520-5520

For business and financial information about Eastern, please contact:

Eastern Bank Investor Relations

265 Franklin Street

Boston, Massachusetts 02110

Email: InvestorRelations@easternbank.com

Telephone: (781) 598-7920

If Cambridge shareholders would like to receive documents before Cambridge’s special meeting, they must request the documents no later than     . If Eastern shareholders would like to receive documents before Eastern’s special meeting, they must request the documents no later than     . Neither Cambridge shareholders nor Eastern shareholders will be charged for any of these documents that they request.

For additional information regarding where you can find information about Eastern and Cambridge, please see the section titled “Where You Can Find More Information” beginning on page 148151 of this joint proxy statement/prospectus. The information contained in this joint proxy statement/prospectus with respect to Eastern and its subsidiaries was provided by Eastern and the information contained in this joint proxy statement/prospectus with respect to Cambridge and its subsidiaries was provided by Cambridge.

For information on submitting your proxy, please refer to the instructions on the enclosed proxy card for the Cambridge special meeting or on the enclosed proxy card for the Eastern special meeting, as applicable.

ABOUT THIS DOCUMENT

This joint proxy statement/prospectus, which forms part of a registration statement on Form S-4 filed by Eastern (File No. 333-275479) with the Securities and Exchange Commission (the “SEC”) constitutes a prospectus of Eastern for purposes of the Securities Act of 1933, as amended, with respect to the shares of Eastern common stock to be issued to Cambridge shareholders in exchange for shares of Cambridge common stock pursuant to the merger agreement. This joint proxy statement/prospectus also constitutes a joint proxy statement for Eastern and Cambridge for purposes of the Securities Exchange Act of 1934, as amended. In addition, it constitutes a notice of meeting with respect to the Eastern and Cambridge special meetings.

You should rely only on the information contained in or incorporated by reference ininto this joint proxy statement/prospectus. No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, this joint proxy statement/prospectus. This joint proxy statement/prospectus is dated             , 2024. You should not assume that the information contained in, or incorporated by reference into, this joint proxy statement/prospectus is accurate as of any date other than that date or the date of such incorporated information. Neither our mailing of this joint proxy statement/prospectus to Eastern shareholders or Cambridge shareholders nor the issuance by Eastern of shares of its common stock to Cambridge shareholders pursuant to the merger agreement will create any implication to the contrary.

This joint proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction in which or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Information contained in this joint proxy statement/prospectus regarding Eastern has been provided by Eastern, and information contained in this joint proxy statement/prospectus regarding Cambridge has been provided by Cambridge.

TABLE OF CONTENTS

DEFINED TERMS

In this joint proxy statement/prospectus, unless the context otherwise requires:

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QUESTIONS AND ANSWERS

The following are some questions that you may have about the merger and the Eastern special meeting or the Cambridge special meeting, and brief answers to those questions. We urge you to read carefully the remainder of this joint proxy statement/prospectus because the information in this section does not provide all of the information that might be important to you with respect to the merger, the Eastern special meeting or the Cambridge special meeting. Additional important information is also contained in the documents incorporated by reference into this joint proxy statement/prospectus. See “Incorporation of Certain Documents by Reference” beginning on page 149152.

Questions and Answers About the Merger

In order to complete the merger, among other things:

Eastern is holding a special meeting of Eastern shareholders (the “Eastern special meeting”) virtually to obtain approval of the Eastern Share Issuance Proposal.

Eastern shareholders will also be asked to approve a proposal to adjourn the Eastern special meeting to solicit additional proxies (i) if there are insufficient votes at the time of the Eastern special meeting to approve the Eastern Share Issuance Proposal or (ii) if adjournment is necessary or appropriate to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Eastern shareholders (the “Eastern Adjournment Proposal”).

Cambridge is holding a virtual special meeting of Cambridge shareholders (the “Cambridge special meeting”) to obtain approval of the Cambridge Merger Proposal.

Cambridge shareholders will also be asked to approve, on an advisory (non-binding) basis, specified compensation that may become payable to the named executive officers of Cambridge in connection with the transactions contemplated by the merger agreement (the “Cambridge Compensation Proposal”), and to approve one or more adjournments of the Cambridge special meeting, if necessary, to permit further solicitation of proxies if there are not sufficient votes at the time of the Cambridge special meeting, or at any adjournment or postponement of that meeting, to approve the Cambridge Merger Proposal (the “Cambridge Adjournment Proposal”).

This document is also a prospectus that is being delivered to holders of Cambridge common stock because, pursuant to the merger agreement, Eastern is offering shares of Eastern common stock to Cambridge shareholders.

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This joint proxy statement/prospectus contains important information about the merger and the other proposals being voted on at the Eastern and Cambridge special meetings. You should read it carefully and in its entirety. The enclosed materials allow you to have your shares of common stock voted by proxy without attending your meeting. Your vote is important and we encourage you to submit your proxy as soon as possible.

Eastern’s common stock is listed on NASDAQ under the trading symbol “EBC.” Eastern will not issue fractional shares of its common stock in the merger, but will instead pay cash for any fractional shares at a price determined by the volume weighted average closing price of Eastern common stock on NASDAQ for the five trading days ending on the fifth trading day immediately preceding the closing date of the merger (the exchange ratio, together with cash received in lieu of fractional shares of Eastern common stock, if any, the “Merger Consideration”).

Each share of Cambridge common stock issued and outstanding immediately prior to the effective time of the merger (the “Effective Time”) (other than shares held as treasury stock or shares owned directly by Eastern) will be converted into the right to receive 4.956 shares of Eastern common stock plus cash in lieu of any fractional shares of Eastern common stock they would have otherwise received in the merger. After completion of the merger, (1) Cambridge will no longer be a public company, (2) Cambridge common stock will be delisted from NASDAQ and will cease to be publicly traded, and (3) the Cambridge common stock will be deregistered under the Securities Exchange Act of 1934, as amended (the “Exchange Act”). After completion of the holdco merger, Cambridge will cease to exist. Holders of Eastern common stock will continue to own their existing shares of Eastern common stock. See the information provided in the section titled “The Merger Agreement—Structure of the Merger” beginning on page 106109 of this joint proxy statement/prospectus and the merger agreement for more information about the merger.

Cambridge RSUs. As of the Effective Time, each time-based vesting restricted stock unit with respect to Cambridge common stock (“Cambridge RSU”) that is then-outstanding will be assumed by Eastern and converted into a time-based vesting restricted stock unit with respect to Eastern common stock (“Eastern

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RSU”), with the number of shares of Eastern common stock subject to such Eastern RSU equal to the product (rounded up to the nearest whole number) of (i) the number of shares of Cambridge common stock subject to such Cambridge RSU as of immediately prior to the Effective Time, multiplied by (ii) the exchange ratio. Eastern RSUs will be subject to the same vesting, termination, deferral and other terms and conditions as the applicable Cambridge RSUs were subject immediately prior to the Effective Time and will include an entitlement to receive on each vesting date of an Eastern RSU a payment equal to the cumulative dollar amount of dividends the holder would have received with respect to the portion of Eastern RSUs then vesting if the holder had been the actual record owner of the underlying Cambridge common stock and, after the Effective Time, Eastern common stock on each dividend record date from the grant date to such vesting date.

Cambridge PRSUs. As of the Effective Time, each performance-based vesting restricted stock unit with respect to Cambridge common stock (“Cambridge PRSU”) that is then-outstanding will be assumed by Eastern and converted into a performance-based vesting restricted stock unit with respect to Eastern common stock (“Eastern PRSU”), with the number of shares of Eastern common stock subject to such Eastern PRSU equal to the product (rounded up to the nearest whole number) of (i) the target number of shares of Cambridge common stock subject to such Cambridge PRSU as of immediately prior to the Effective Time, multiplied by (ii) the exchange ratio. Eastern PRSUs will be subject to the same vesting (including performance metrics), termination, deferral and other terms and conditions as the applicable Cambridge PRSUs were subject immediately prior to the Effective Time.

To the extent not settled prior to the Effective Time, the Cambridge PRSUs granted in 2021 will be settled as soon as practicable following the Effective Time based upon Cambridge’s actual performance through December 31, 2023.

For the Cambridge PRSUs granted in 2022 or 2023, as soon as practicable after the Effective Time, Eastern will offer to each holder of a Cambridge PRSU granted in 2022 or 2023 that was converted into a Eastern PRSU the opportunity to exchange such Eastern PRSU for a Eastern RSU either (i) with respect to a number of shares of Eastern common stock equal to the number of shares of Eastern common stock subject to such Eastern PRSU as of the Effective Time (with (1) the same termination terms, deferral elections and other terms and restrictions as the applicable Eastern PRSU was subject to as of the Effective Time (excluding the performance conditions), (2) same settlement date as the Eastern PRSU that it replaces and (3) no dividend equivalent rights with respect thereto) or (ii) with respect to a number of shares of Eastern common stock determined based on measurement of actual performance determined as of the Effective Time (with (1) the same termination terms, deferral elections and other terms and restrictions as the applicable Eastern PRSU was subject to as of the Effective Time, except that such Eastern RSU is solely subject to future time-based vesting, (2) the same settlement date as the Eastern PRSU that it replaces and (3) the same entitlement to receive on the vesting date of the Eastern RSU a payment equal to the cumulative dollar amount of dividends the holder would have received with respect to the applicable Cambridge PRSUs if the holder had been the actual record owner of the underlying Cambridge common stock on each dividend record date from the grant date through the Effective Time).

Cambridge RSAs. As of the Effective Time, each award of restricted shares of Cambridge common stock (“Cambridge RSA”) that is then-outstanding will be assumed and converted into an award of restricted shares of Eastern common stock (“Eastern RSA”), with the number of restricted shares of Eastern common stock subject to such Eastern RSA equal to the product (rounded up to the nearest whole number) of (i) the number of shares of Cambridge common stock subject to such Cambridge RSA as of immediately prior to the Effective Time, multiplied by (ii) the exchange ratio. Eastern RSAs will be subject to the same vesting, termination, and other terms and conditions as the applicable Cambridge RSAs were subject immediately prior to the Effective Time.

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Questions and Answers About the Special Meeting of Cambridge shareholdersShareholders

Even if you plan to attend the Cambridge special meeting, Cambridge recommends that you vote your shares in advance as described below so that your vote will be counted if you later decide not to or become unable to attend the applicable special meeting.

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In order to complete the merger, among other things, Cambridge shareholders must approve the Cambridge Merger Proposal. Neither of the approval of the Cambridge Compensation Proposal nor the Cambridge Adjournment Proposal is a condition to the obligations of Eastern or Cambridge to complete the merger.

In considering the recommendations of the Cambridge board of directors, Cambridge shareholders should be aware that Cambridge directors and executive officers may have interests in the merger that are different from, or in addition to, the interests of Cambridge shareholders generally. See the section titled “The Merger—Interests of Cambridge’s Executive Officers and Directors in the Merger” beginning on page 9193 of this joint proxy statement/prospectus for a complete description of these interests.

Attendance at the special meeting is not required to vote. See below and the section titled “The Special Meeting of Cambridge Shareholders— Voting of Proxies” beginning on page 45 of this joint proxy statement/prospectus for instructions on how to vote your shares of Cambridge common stock without attending the Cambridge special meeting.

Cambridge Proposal 2: Cambridge Compensation Proposal. Approval of the Cambridge Compensation Proposal requires the affirmative vote of a majority of the votes cast by Cambridge shareholders. Accordingly, assuming a quorum is present at the Cambridge special meeting, an abstention or a broker non-vote or other failure to vote will have no effect on the outcome of the Cambridge Compensation Proposal.

Cambridge Proposal 3: Cambridge Adjournment Proposal. Approval of the Cambridge Adjournment Proposal requires the affirmative vote of a majority of the votes cast by Cambridge shareholders. Accordingly, assuming a quorum is present at the Cambridge special meeting, an abstention or a broker non-vote or other failure to vote will have no effect on the outcome of the Cambridge Adjournment Proposal.

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Proxies submitted by mail must be received by the close of business on                     . Proxies submitted by telephone or through the Internet must be received by         , Eastern Time, on                    .

Shares Held in Brokerage Accounts: If you hold your shares of Cambridge common stock in street name (that is, you hold your shares of Cambridge common stock through a broker, bank or other holder of record), your bank, broker or other holder of record will forward proxy materials and voting instructions that you must follow in order to vote your shares of Cambridge common stock. You may receive more than one proxy card if your shares of Cambridge common stock are registered in different names or are held in more than one account. A broker, bank or other holder of record will not be able to vote your shares at the Cambridge special meeting without first receiving instructions from you on how to vote.

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Shares Held in the Cambridge ESOP Plan: If you are a participant in the Cambridge ESOP and indirectly hold shares of Cambridge common stock through the Cambridge ESOP, you are generally entitled to direct the trustees of the Cambridge ESOP’s related trust as to the manner in which any of the common stock allocated to your Cambridge ESOP account is voted by following the separate voting instructions provided by the trustees of the Cambridge ESOP. Your Cambridge ESOP vote authorization form must be received by     , Eastern Time, on     . The telephonic and internet voting cutoff for providing your ESOP vote authorization is     , Eastern Time, on     . Under the terms of the Cambridge ESOP, the trustees of the Cambridge ESOP vote all shares held by the Cambridge ESOP, but each Cambridge ESOP participant may direct the trustees how to vote the shares of Cambridge common stock allocated to his or her account. The trustees of the Cambridge ESOP will, as instructed by Cambridge, vote all unallocated shares of Cambridge common stock held by the Cambridge ESOP, if any, for each of the Cambridge Merger Proposal, the Cambridge Compensation Proposal and the Cambridge Adjournment Proposal and will vote allocated shares for which no voting instructions are received in the same proportion as shares for which it has received timely voting instructions, so long as such vote is solely in the interests of participants and beneficiaries and in accordance with the requirements of the Employee Retirement Income Security Act of 1974, as amended.

Cambridge Bancorp

Attention: Corporate Secretary

1336 Massachusetts Avenue

Cambridge, Massachusetts 02138

Participants in the Cambridge ESOP may revoke their instructions to the trustees of the Cambridge ESOP with respect to voting of the shares of Cambridge common stock held in their Cambridge ESOP account by submitting to the trustees of the Cambridge ESOP a signed instruction card bearing a later date, but such new instruction card must be received by the trustees of the Cambridge ESOP on or prior to the last date for submission of such instructions with respect to the Cambridge special meeting designated in the separate voting instructions provided by the trustees of the Cambridge ESOP.

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Alliance Advisors, LLC

200 Broadacres Drive, 3rd Floor

Bloomfield, NJ 07003

844-984-3713 (toll free)

CATC@allianceadvisors.com

Questions and Answers About the Special Meeting of Eastern shareholdersShareholders

In order to complete the merger, among other things, Eastern shareholders must approve the Eastern Share Issuance Proposal. The approval of the Eastern Adjournment Proposal is not a condition to the obligations of Eastern or Cambridge to complete the merger.

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Eastern Proposal 2: Eastern Adjournment Proposal. Approval of the Eastern Adjournment Proposal requires the affirmative vote of a majority of the votes cast by Eastern shareholders at the Eastern special meeting. Accordingly, assuming a quorum is present at the Eastern special meeting, an abstention or a broker non-vote or other failure to vote will have no effect on the outcome of the Eastern Adjournment Proposal.

Proxies submitted by mail must be received by the close of business on .Monday, January 8, 2024. Proxies submitted by telephone or through the Internet must be received by                    , Eastern Time, on                     .

Shares Held in Brokerage Accounts: If you hold your shares of Eastern common stock in street name (that is, you hold your shares of Eastern common stock through a broker, bank or other holder of record), your bank, broker or other holder of record will forward proxy materials and voting instructions that you must follow in order to vote your shares of Eastern common stock. You may receive more than one proxy card if your shares of Eastern common stock are registered in different names or are held in more than one account. A broker, bank or other holder of record will not be able to vote your shares at the Eastern special meeting without first receiving instructions from you on how to vote.

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Shares Held in the Eastern 401(k) Plan and ESOP. If you are a participant in Eastern’s 401(k) Plan or ESOP and indirectly hold shares of Eastern common stock through the Eastern 401(k) Plan or ESOP, the trustee or administrator votes shares of Eastern common stock held by the plan. However, you may direct the trustee or administrator on how to vote the shares of Eastern common stock allocated to your plan account. To do so, your voting instructions must be received by no later than                 . If you do not provide instructions by that date, the respective plan trustees or administrators will vote your shares in accordance with the terms of the respective plans. Please note that plan participants must provide voting instructions by the specified deadline (                ), which is earlier than the voting deadline for shareholders of record.

Additional information on attending the Eastern special meeting can be found under the section titled “The Special Meeting of Eastern Shareholders—Attending the Eastern Special Meeting” on page 51.51 of this joint proxy statement/prospectus.

c/o Corporate Secretary, Eastern Bankshares, Inc.

265 Franklin Street

Boston, Massachusetts 02110

Participants in the Eastern 401(k) Plan may revoke their instructions to the Eastern 401(k) Plan administrator with respect to voting of the shares of Eastern common stock held in their Eastern 401(k) Plan account by submitting to the Eastern 401(k) Plan administrator a signed instruction card bearing a later date, provided that such new instruction card must be received by the Eastern 401(k) Plan administrator on or prior to the last date for submission of such instructions with respect to the Eastern special meeting designated in the separate voting instructions provided by the Eastern 401(k) Plan administrator.

Participants in the Eastern ESOP may revoke their instructions to the Eastern ESOP trustee with respect to voting of the shares of Eastern common stock held in their Eastern ESOP account by submitting to the Eastern ESOP trustee a signed instruction card bearing a later date, provided that such new instruction card must be received by the Eastern ESOP trustee on or prior to the last date for submission of such instructions with respect to the Eastern special meeting designated in the separate voting instructions provided by the Eastern ESOP trustee.

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The Eastern proxy card will instruct the persons named on the Eastern proxy card to vote the Eastern Shareholder’sshareholder’s shares of Eastern common stock at the Eastern special meeting as the Shareholdershareholder directs. If an Eastern Shareholdershareholder signs, dates and sends in an Eastern proxy card and does not indicate how the shareholder wishes to vote, the proxy will be voted “FOR” both of the Eastern special meeting proposals.

Eastern Bank Investor Relations

265 Franklin Street

Boston, Massachusetts 02110

(781) 598-7920

InvestorRelations@easternbank.com

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RISK FACTORS

In addition to the other information included in this joint proxy statement/prospectus, including the matters addressed under “Cautionary Statement Regarding Forward-Looking Statements,” Cambridge shareholders and Eastern shareholders should carefully consider the following risks before deciding whether to vote for approval of the merger agreement. In addition, Cambridge shareholders should read and consider the risks associated with the business of Eastern because these risks will relate to the combined company. Certain of these risks with respect to the business of Eastern can be found in Eastern’s annual report on Form 10-K for the fiscal year ended December 31, 2022 (and, together with Eastern’s recast consolidated financial statements for the year ended December 31, 2022 filed as an exhibit to Eastern’s Current Report on Form 8-K dated November 8, 2023, “Eastern’s 2022 10-K and Recast Disclosures”), and quarterly report on Form 10-Q for the quarter ended September 30, 2023, which reports are incorporated by reference into this joint proxy statement/prospectus. You should also consider the other information in this joint proxy statement/prospectus and the other documents incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 148151 and “Incorporation of Certain Documents by Reference” beginning on page 149.152 of this joint proxy statement/prospectus.

Risks Related to the Merger

Because the market price of Eastern common stock will fluctuate, Cambridge shareholders cannot be sure of the trading price of the Merger Consideration they will receive.

In the merger, each share of Cambridge common stock issued and outstanding immediately prior to the Effective Time, except for certain shares owned by Eastern or Cambridge, will be converted into the right to receive 4.956 shares of Eastern common stock. This exchange ratio is fixed and will not be adjusted for changes in the market price of either Eastern common stock or Cambridge common stock. Changes in the price of Eastern common stock between now and the time of the merger will affect the value that Cambridge shareholders will receive in the merger. Neither Eastern nor Cambridge is permitted to terminate the merger agreement as a result of any increase or decrease in the market price of Eastern common stock or Cambridge common stock.

Stock price changes may result from a variety of factors, including general market and economic conditions, changes in Cambridge’s and Eastern’s businesses, operations and prospects, the prices of securities in global financial markets, including market prices of Cambridge, Eastern and other banking companies and regulatory considerations, many of which are beyond Cambridge’s and Eastern’s control. Therefore, at the time of the Eastern special meeting and the Cambridge special meeting, Eastern shareholders and Cambridge shareholders will not know the market value of the consideration that Cambridge shareholders will receive at the Effective Time. You should obtain current market quotations for shares of Eastern common stock and for shares of Cambridge common stock.

The market price of Eastern common stock after the merger may be affected by factors different from those currently affecting the shares of Cambridge common stock or Eastern common stock.

In the merger, Cambridge shareholders will become Eastern shareholders. Eastern’s business differs from that of Cambridge. Accordingly, the results of operations of Eastern and the market price of Eastern common stock after the completion of the merger may be affected by factors different from those currently affecting the independent results of operations of each of Eastern and Cambridge. For a discussion of the businesses of Eastern and Cambridge and of certain factors to consider in connection with those businesses, see the documents incorporated by reference in this joint proxy statement/prospectus and referred to under “Where You Can Find More Information” beginning on page 148.151 of this joint proxy statement/prospectus.

The fairness opinions rendered to the boards of directors of Cambridge and Eastern by the parties’ respective financial advisors prior to the signing of the merger agreement do not reflect changes in events or circumstances occurring after the respective dates of the opinions.

The opinions of BofA Securities, financial advisor to Cambridge, and of J.P. Morgan, financial advisor to Eastern, were each delivered on and dated September 19, 2023. Neither opinion reflects changes that may occur

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or may have occurred after the date on which it was delivered, including changes to the operations and prospects of Cambridge or Eastern, changes in general market and economic conditions or other changes. Any of these changes may alter the relative value of Cambridge or Eastern or the prices of shares of Cambridge common stock or Eastern common stock by the time the merger is completed. The opinions do not speak as of the date the merger will be completed or as of any date other than the dates of the respective opinions. For a description of the opinion of Cambridge’s financial advisor, please see “The Merger—Opinion of Cambridge’s Financial Advisor.” For a description of the opinion of Eastern’s financial advisor, please see “The Merger—Opinion of Eastern’s Financial Advisor.”

Cambridge will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on Cambridge and, consequently, on Eastern. These uncertainties may impair Cambridge’s ability to attract, retain and motivate key personnel until the merger is consummated, and could cause customers and others that deal with Cambridge to seek to change existing business relationships with Cambridge. Retention of senior Cambridge employees who are not anticipated to have a role with the combined company may be challenging during the pendency of the merger, as prospective severance or retention payments may not provide a sufficient incentive for them to remain with Cambridge. If key employees depart because of issues relating to the uncertainty or difficulty of integration or a desire not to remain with Eastern, Eastern’s business following the merger could be harmed. In addition, the merger agreement restricts Cambridge from taking certain actions without the consent of Eastern until the merger occurs. These restrictions may prevent Cambridge from pursuing attractive business opportunities that may arise prior to the completion of the merger. Please see the section titled “The Merger Agreement – Covenants and Agreements – Conduct of Business Prior to the Consummation of the Merger” on page 112115 of this joint proxy statement/prospectus for a description of the restrictive covenants to which Cambridge is subject.

Eastern may fail to realize all of the anticipated benefits of the merger, particularly if the integration of Eastern’s and Cambridge’s businesses is more difficult than expected.

The success of the merger will depend, in part, on Eastern’s ability to successfully combine the businesses of Eastern and Cambridge. Eastern may fail to realize some or all of the anticipated benefits of the transaction if the integration process takes longer or is more costly than expected. Furthermore, any number of unanticipated adverse occurrences for either the business of Cambridge or Eastern may cause Eastern to fail to realize some or all of the expected benefits. The integration process could result in the loss of key employees, the disruption of each company’s ongoing businesses or inconsistencies in standards, controls, procedures and policies that adversely affect our ability to maintain relationships with clients, customers, depositors and employees or to achieve the anticipated benefits of the merger. Each of these issues might adversely affect Eastern, Cambridge or both during the transition period, resulting in adverse effects on Eastern following the merger. As a result, revenues may be lower than expected or costs may be higher than expected and the overall benefits of the merger may not be as great as anticipated.

Some of the Cambridge executive officers and directors may have interests and arrangements that may have influenced their decisions to support and recommend that you approve the merger.

The interests of executive officers and directors of Cambridge are different from, or in addition to, the interests of Cambridge’s other shareholders generally. These interests include (i) assumption and conversion of Cambridge equity awards into Eastern equity awards; (ii) rights of certain executive officers under their existing Cambridge employment and change in control agreements; (iii) rights of certain executive officers under offer letters, severance benefits agreements and change in control agreements with Eastern Bank executed in connection with the merger agreement; (iv) rights under the Cambridge ESOP; (v) prorated retainer payable to members of the board of directors of Cambridge; and (vi) rights to continued indemnification and insurance coverage by Eastern after the merger for acts and omissions occurring before the merger. In addition, upon the completion of the merger, Mr. Sheahan will become an Eastern director and Chief Executive Officer, reporting to

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Robert Rivers, and three additional Cambridge directors selected by Eastern in consultation with Cambridge will become members of the board of directors of Eastern and Eastern Bank following the closing. The board of directors of Cambridge was aware of these interests and considered them among other matters, in approving the merger agreement and related transactions.

See the section titled “The Merger—Interests of Cambridge’s Executive Officers and Directors in the Merger” beginning on page 9193 of this joint proxy statement/prospectus for a more complete description of these interests.

Eastern may be unable to retain Eastern and/or Cambridge personnel successfully after the merger is completed.

The success of the merger will depend in part on Eastern’s ability to retain the talents and dedication of key employees currently employed by Eastern and Cambridge. It is possible that these employees may decide not to remain with Eastern or Cambridge, as applicable, while the merger is pending or with Eastern after the merger is completed. If Eastern and Cambridge are unable to retain key employees, including management, who are critical to the successful integration and future operations of the companies, Eastern and Cambridge could face disruptions in their operations, loss of existing customers, loss of key information, expertise or know-how and unanticipated additional recruitment costs. In addition, following the merger, if key employees terminate their employment, Eastern’s business activities may be adversely affected, and management’s attention may be diverted from successfully integrating Eastern and Cambridge to hiring suitable replacements, all of which may cause Eastern’s business to suffer. In addition, Eastern and Cambridge may not be able to locate or retain suitable replacements for any key employees who leave either company.

The unaudited pro forma condensed combined financial statements included in this joint proxy statement/prospectus are preliminary and the actual purchase price as well as the actual financial condition and results of operations of Eastern after the merger may differ materially.

The unaudited pro forma condensed combined financial statements in this joint proxy statement/prospectus is presented for illustrative purposes only and is not necessarily indicative of what Eastern’s actual financial condition or results of operations would have been had the merger been completed on the dates indicated. The unaudited pro forma condensed combined financial statements reflect adjustments, which are based upon preliminary estimates, to record the Cambridge identifiable assets acquired and liabilities assumed at fair value and the resulting goodwill recognized. The purchase price allocation reflected in this document is preliminary, and final allocation of the purchase price will be based upon the actual purchase price and the fair value of the assets and liabilities of Cambridge as of the date of the completion of the merger. Accordingly, the actual purchase price may vary significantly from the purchase price used in preparing the unaudited pro forma combined consolidated financial information in this document. Accordingly, the final acquisition accounting adjustments may differ materially from the pro forma adjustments reflected in this document. For more information, please see “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 35.35 of this joint proxy statement/prospectus.

Eastern and Cambridge have incurred and expect to continue to incur significant costs related to the merger and integration.

Eastern and Cambridge have incurred and expect to incur significant, non-recurring costs in connection with negotiating the merger agreement and closing the merger. In addition, Eastern will incur integration costs following the completion of the merger as Eastern integrates the Cambridge business, including facilities and systems consolidation costs and employment-related costs. Eastern and Cambridge will also incur significant legal, financial advisory, accounting, banking and consulting fees, fees relating to regulatory filings and notices, SEC filing fees, printing and mailing fees and other costs associated with the merger. Some of these costs are payable regardless of whether the merger is completed. See “The Merger Agreement—Fees and Expenses” beginning on page 125.128 of this joint proxy statement/prospectus.

Eastern and Cambridge may also incur additional costs to maintain employee morale and to retain key employees. There can be no assurances that the expected benefits and efficiencies related to the integration of the businesses will be realized to offset these transaction and integration costs over time.

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The merger agreement limits Cambridge’s ability to pursue alternatives to the merger.

The merger agreement contains provisions that limit Cambridge’s ability to solicit, initiate, encourage or take any actions to facilitate competing third-party proposals to acquire all or substantially all of Cambridge. These provisions, which include a $21.0 million termination fee, payable under certain circumstances, might discourage a potential competing acquiror that might have an interest in acquiring all or substantially all of Cambridge from considering or proposing that acquisition even if it were prepared to pay consideration with a higher per share market price than that proposed in the merger, or might result in a potential competing acquiror proposing to pay a lower per share price to acquire Cambridge than it might otherwise have proposed to pay.

Regulatory approvals may not be received, may take longer to receive than expected or may impose burdensome conditions that are not presently anticipated.

Before the merger may be completed, certain approvals or consents must be obtained from the various bank regulatory and other authorities of the United States, the Commonwealth of Massachusetts and the State of New Hampshire. These governmental entities, including the Federal Reserve Board, the FDIC, the Massachusetts Division of Banks and the New Hampshire Banking Department, may impose conditions on the completion of the merger or require changes to the terms of the merger. While Eastern and Cambridge do not currently expect that any such conditions or changes would be imposed, there can be no assurance that they will not be, and such conditions or changes could have the effect of delaying completion of the merger or imposing additional costs on or limiting the revenues of Eastern following the merger, any of which might have a material adverse effect on Eastern following the merger. Eastern is not obligated to complete the merger if the regulatory approvals received in connection with the completion of the merger include any conditions or restrictions that would constitute a “Burdensome Condition” as defined in the merger agreement.

Although the parties believe they have filed all applications and notice materials and have submitted all requested supplemental information necessary to obtain these regulatory approvals or confirmations in accordance with applicable law, there can be no assurance as to whether the regulatory approvals will be received or the timing of the approvals. For more information, see the section titled “The Merger—Regulatory Approvals Required to Complete the Merger” of this joint proxy statement/prospectus beginning on page 101.104 of this joint proxy statement/prospectus.

If the merger is not consummated by September 19, 2024, either Eastern or Cambridge may choose not to proceed with the merger.

Either Eastern or Cambridge may terminate the merger agreement if the merger has not been completed by September 19, 2024, unless the failure of the merger to be completed has resulted from the failure of the party seeking to terminate the merger agreement to perform its obligations.

The shares of Eastern common stock to be received by Cambridge shareholders as a result of the merger will have different rights from the shares of Cambridge common stock.

The rights associated with Cambridge common stock are different from the rights associated with Eastern common stock. See the section of this joint proxy statement/prospectus titled “Comparison of Rights of Shareholders of Eastern and Shareholders of Cambridge” beginning on page 134137 of this joint proxy statement/prospectus for a discussion of the different rights associated with Eastern common stock.

Shareholders of Cambridge will have a reduced ownership and voting interest after the merger and will exercise less influence over management.

As a result of the merger, the percentage ownership of every Cambridge shareholder in the combined company will be smaller than the Cambridge shareholder’s percentage ownership of Cambridge prior to the merger. Eastern estimates that upon completion of the merger, current Cambridge shareholders will own approximately 18.2% of the outstanding shares of Eastern common stock, and current Eastern shareholders will own approximately 81.8% of the outstanding shares of Eastern common stock.

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Failure to complete the merger could negatively impact the future business and financial results of Cambridge.

If the merger is not completed, the ongoing business of Cambridge may be adversely affected and Cambridge will be subject to several risks, including the following:

In addition, if the merger is not completed, Cambridge may experience negative reactions from its customers and employees. Cambridge also could be subject to litigation related to any failure to complete the merger or to enforcement proceedings commenced against Cambridge to perform its obligations under the merger agreement. If the merger is not completed, Cambridge cannot assure its shareholders that the risks described above will not materialize and will not materially affect the business and financial results of Cambridge.

Cambridge shareholders will not have dissenters’ rights in the merger.

Dissenters’ rights are statutory rights that, if applicable under law, enable shareholders to dissent from an extraordinary transaction, such as a merger, and to demand that the corporation pay the fair value for their shares as determined by a court in a judicial proceeding instead of receiving the consideration offered to shareholders in connection with the extraordinary transaction. Under the MBCA, a shareholder may not dissent from a merger as to shares that are listed on a national securities exchange at the record date fixed to determine the shareholders entitled to receive notice of the meeting of shareholders to vote upon the agreement of merger or consolidation.

Because Cambridge common stock is listed on NASDAQ, a national securities exchange, and is expected to continue to be so listed on the record date, and because the merger otherwise satisfies the foregoing requirements of the MBCA, holders of Cambridge common stock will not be entitled to dissenters’ rights in the merger with respect to their shares of Cambridge common stock.

Eastern will be able to issue additional shares of its common stock in the future, which may adversely affect the market price of Eastern common stock and dilute the holdings of existing shareholders.

In the future, Eastern may issue additional shares of Eastern common stock in connection with another acquisition, to increase its capital resources or if Eastern’s or Eastern Bank’s capital ratios fall below or near regulatory required minimums in effect from time to time. Additional common stock offerings may dilute the holdings of Eastern’s then existing shareholders or reduce the market price of Eastern common stock, or both. Eastern may also issue shares of Eastern preferred stock, which may be viewed as having adverse effects upon the holders of Eastern common stock.

Since the initial filing on November 13, 2023 of the registration statement of which this joint proxy statement/prospectus is a part, Eastern and the Eastern board of directors have received four Demand Letters from certain Eastern shareholders, which could result in litigation related to the merger being filed against Eastern, the Eastern board of directors and/or Cambridge and the Cambridge board of directors, and additional demand letters may be received or litigation may be filed against them, which could prevent or delay the

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completion of the merger or otherwise negatively impact the business and operations of Eastern and Cambridge.

Since the initial filing on November 13, 2023 of the registration statement of which this joint proxy statement/prospectus is a part, Eastern has received Demand Letters from four purported Eastern shareholders generally alleging that the registration statement filed with the SEC on November 13, 2023 omits material information in violation of the federal securities laws. The shareholders have demanded disclosure of certain additional information pertaining to certain financial projections for each of Eastern and Cambridge, certain information with respect to J.P. Morgan’s analysis and opinion, and other requested disclosures.

The shareholders who provided the Demand Letters, or other shareholders, may initiate litigation against Eastern, the Eastern board of directors, Cambridge and the Cambridge board of directors, and it is possible that Eastern, the Eastern board of directors, Cambridge or the Cambridge board of directors may receive further demand letters alleging wrongdoing with respect to the merger.

One of the conditions to the closing is that no order, injunction or decree issued by any court or governmental entity of competent jurisdiction or other legal restraint preventing the consummation of the merger, the holdco merger, the bank merger or any of the other transactions contemplated by the merger agreement be in effect. If any plaintiff were successful in obtaining an injunction prohibiting Eastern or Cambridge from completing the merger, the holdco merger, the bank merger or any of the other transactions contemplated by the merger agreement, then such injunction may delay or prevent the effectiveness of the merger and could result in significant costs to Eastern and/or Cambridge, including any cost associated with the indemnification of directors and officers of each company. Eastern and Cambridge may also incur costs in connection with the defense or settlement of any shareholder lawsuit filed in connection with the merger. Such litigation could have an adverse effect on the financial condition and results of operations of Eastern and Cambridge and could prevent or delay the completion of the merger.

The merger and related transactions are subject to approval by shareholders of both Eastern and Cambridge

The merger cannot be completed unless (i) Cambridge’s shareholders approve the merger agreement by the affirmative vote of the holders of at least two-thirds of the Cambridge common stock outstanding and entitled to vote on the proposal and (ii) Eastern’s shareholders approve the issuance of Eastern common stock to holders of Cambridge common stock pursuant to the merger agreement, by the affirmative vote of a majority of the votes cast by the holders of Eastern common stock at the Eastern special meeting. If shareholder approval is not obtained by Eastern’s or Cambridge’s shareholders, the merger and related transactions cannot be completed.

Risks Related to Eastern’s Business

You should read and consider risk factors specific to Eastern’s business that will also affect Eastern after the merger. These risks are described in the sections titled “Risk Factors” in Eastern’s 2022 10-K and Recast Disclosures, Quarterly Report on Form 10-Q for the quarter ended September 30, 2023, and in other documents incorporated by reference into this joint proxy statement/prospectus. Please see the section titled “Where You Can Find More Information” beginning on page 148151 of this joint proxy statement/prospectus for the location of information incorporated by reference into this joint proxy statement/prospectus.

Risks Related to Cambridge’s Business

You should read and consider risk factors specific to Cambridge’s business that will also affect Eastern following the completion of the merger. These risks are described in the sections titled “Risk Factors” in Cambridge’s Annual Report on Form 10-K for the year ended December 31, 2022 and in other documents incorporated by reference into this joint proxy statement/prospectus. Please see the section titled “Where You Can Find More Information” beginning on page 148151 of this joint proxy statement/prospectus for the location of information incorporated by reference into this joint proxy statement/prospectus.

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RECENT DEVELOPMENTS

On October 31, 2023, Eastern completed the sale of Eastern Bank’s insurance operations, which conducted business under the name Eastern Insurance Group LLC (“Eastern Insurance”) to a wholly owned subsidiary of Arthur J. Gallagher & Co. (“Gallagher”) pursuant to a definitive asset purchase agreement dated September 19, 2023. Gallagher paid Eastern a gross purchase price of $515 million in cash, which is subject to customary net working capital adjustments. Gallagher’s recourse to Eastern for post-closing indemnification claims is capped under the asset purchase agreement at approximately $1.3 million, subject to customary exceptions.

Eastern Insurance, which has been a wholly owned subsidiary of Eastern Bank since 2002, acted as an independent agent in offering commercial property and casualty, personal lines products and employee benefits consulting services to both personal and commercial customers. As of October 31, 2023, Eastern Insurance had approximately 380 full-time equivalent employees and operated through 21 non-branch offices in eastern Massachusetts, one office in Keene, New Hampshire, and one office in Providence, Rhode Island.

Eastern, Eastern Bank and Eastern Insurance agreed in the asset purchase agreement that through October 31, 2026, none of them, nor any entity that is an affiliate of any of them, will, directly or indirectly, own, invest in, provide financial resources to, operate, join, manage, control, undertake, participate in, engage in or assist others participating in or engaging in the business of acting as an independent agent in offering personal, business and employee benefits insurance products and captive solutions and related risk management services to individual and commercial clients, and providing professional advice related to health care cost management, employee engagement and executive services to such clients, or in any other activity for any business division that directly competes with Eastern’s Insurance’s business, subject to customary exceptions for permitting Eastern to provide banking, wealth management or similar financial products or services to any person who engages in any business or activity that is competitive with Eastern Insurance’s business or to own as a passive investment less than 4% of any class of securities of an entity that competes with Eastern Insurance’s business.

As required by GAAP and applicable SEC rules, Eastern, in September 2023, following the approval by Eastern’s board of directors of a definitive agreement with Gallagher for the Asset Sale, classified Eastern Insurance’s operations as held for sale and discontinued in all material respects, and accordingly Eastern recast prior period information to remove the results of operations of the disposed business from the results of the Company’s continuing operations. Eastern first reported the Eastern Insurance operations as discontinued in Eastern’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2023, filed with the SEC on November 3, 2023. In a Current Report on Form 8-K filed with the SEC on November 8, 2023, Eastern filed a recast presentation of its financial information for all periods presented in Eastern’s Annual Report on Form 10-K for the year ended December 31, 2022 to present the Eastern Insurance operations as discontinued in all material respects and also recast certain other disclosures contained in that Form 10-K (including the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” or MD&A) to present operations related to the Eastern Insurance business as discontinued operations. See “Where You Can Find More Information” beginning on page 148.151 of this joint proxy statement/prospectus.

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UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION

The following unaudited pro forma condensed combined financial information combines the historical consolidated financial position and results of operations of Eastern and Cambridge as an acquisition by Eastern of Cambridge. The merger agreement provides that each holder of Cambridge common stock receive 4.956 shares of Eastern’s common stock for each share of Cambridge common stock held. The transaction is intended to qualify as a tax-free reorganization for federal income tax purposes and provide Cambridge shareholders with a tax-free exchange for Eastern’s common stock consideration they will receive in the merger. Eastern anticipates issuing approximately 39.4 million shares of its common stock in the merger.

In addition, the following unaudited pro forma condensed financial information reflects the statements of income for the nine months ended September 30, 2023, and for the year ended December 31, 2022, as if the merger of Cambridge into Eastern and the sale (the “Asset Sale”) of Eastern Bank’s insurance operations, which conducted business under the name Eastern Insurance Group LLC, to Arthur J. Gallagher Risk Management Services, LLC had occurred on January 1, 2022. The unaudited pro forma condensed balance sheet as of September 30, 2023, assumes that the merger of Cambridge into Eastern and the Asset Sale occurred as of September 30, 2023.

The unaudited pro forma condensed combined financial information have been prepared to give effect to the following:

The following unaudited pro forma condensed combined financial information and related notes are based on and should be read in conjunction with (i) the historical audited consolidated financial statements of Eastern and the related notes and Management’s Discussion and Analysis of Financial Condition and Results of Operations included in Eastern’s 2022 10-K and Recast Disclosures, and the historical unaudited condensed consolidated financial statements of Eastern and the related notes included in Eastern’s Quarterly Report on Form 10-Q for the nine month period ended September 30, 2023 and (ii) the historical audited consolidated financial statements of Cambridge and the related notes included in Cambridge’s Annual Report on Form 10-K for the year ended December 31, 2022, and the historical unaudited consolidated financial statements of Cambridge and the related notes included in Cambridge’s Quarterly Report on Form 10-Q for the nine month period ended September 30, 2023.

The unaudited pro forma condensed combined financial information is provided for illustrative information purposes only. The unaudited pro forma condensed combined financial information is not necessarily, and should not be assumed to be, an indication of the actual results that would have been achieved had the merger and Asset Sale been completed as of the dates indicated or that may be achieved in the future. The pro forma financial information has been prepared by Eastern in accordance with Regulation S-X Article 11, Pro Forma Financial Information, as amended by the final rule, Amendments to Financial Disclosures About Acquired and Disposed Businesses, as adopted by the SEC on May 21, 2020.

The merger-related pro forma adjustments included to arrive at the unaudited pro forma condensed combined financial information reflect estimated fair values of the assets and liabilities of Cambridge based upon historical information as of June 30, 2023, the most recent practicable date. Eastern has not performed the detailed valuation analysis necessary to determine the fair market values of Cambridge’s assets to be acquired and liabilities to be assumed, and accordingly, estimates have been used. Therefore, the unaudited pro forma condensed combined financial data includes an estimated allocation of the purchase price. The pro forma

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adjustments included in this joint proxy statement/prospectus are subject to change depending on changes in interest rates and the components of assets and liabilities, and as additional information becomes available and additional analyses are performed. The final allocation of the purchase price will be determined after the merger is completed and after completion of thorough analyses to determine the fair value of Cambridge’s tangible and identifiable intangible assets and liabilities as of the date the merger is completed. Increases or decreases in the fair values of the net assets as compared with the information shown in the unaudited pro forma condensed combined financial data may change the amount of the purchase price allocated to goodwill and other assets and liabilities and may affect Eastern’s statement of operations due to adjustments in yield and/or amortization of the adjusted assets or liabilities. Any changes to Cambridge’s shareholders’ equity, including results of operations and certain balance sheet changes from September 30, 2023 through the date the merger is completed will also change the purchase price allocation, which may include the recording of a lower or higher amount of goodwill. The final adjustments may be materially different from the unaudited pro forma adjustments presented in this joint proxy statement/prospectus. Among other final adjustments, the actual gain on sale of the Eastern Insurance operations, net of tax, will be calculated as of October 31, 2023 when the Asset Sale was completed and may be different than the estimated gain on sale of the Eastern Insurance operations, net of tax, reflected on the pro forma balance sheet, which was calculated based upon the net assets of the discontinued Eastern Insurance operations as of September 30, 2023. Eastern does not expect any such difference to be material to its financial condition or results of operations.

The unaudited pro forma condensed combined financial information also does not consider any potential effects of changes in market conditions on revenues, expense efficiencies, asset dispositions, and share repurchases, among other factors. The pro forma adjustments have been made solely for the purpose of providing the unaudited pro forma condensed combined financial information.

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UNAUDITED PRO FORMA CONDENSED COMBINED BALANCE SHEETCambridge Bancorp

AsAttention: Corporate Secretary

1336 Massachusetts Avenue

Cambridge, Massachusetts 02138

Participants in the Cambridge ESOP may revoke their instructions to the trustees of the Cambridge ESOP with respect to voting of the shares of Cambridge common stock held in their Cambridge ESOP account by submitting to the trustees of the Cambridge ESOP a signed instruction card bearing a later date, but such new instruction card must be received by the trustees of the Cambridge ESOP on or prior to the last date for submission of such instructions with respect to the Cambridge special meeting designated in the separate voting instructions provided by the trustees of the Cambridge ESOP.

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Alliance Advisors, LLC

200 Broadacres Drive, 3rd Floor

Bloomfield, NJ 07003

844-984-3713 (toll free)

CATC@allianceadvisors.com

Questions and Answers About the Special Meeting of Eastern Shareholders

In order to complete the merger, among other things, Eastern shareholders must approve the Eastern Share Issuance Proposal. The approval of the Eastern Adjournment Proposal is not a condition to the obligations of Eastern or Cambridge to complete the merger.

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Eastern Proposal 2: Eastern Adjournment Proposal. Approval of the Eastern Adjournment Proposal requires the affirmative vote of a majority of the votes cast by Eastern shareholders at the Eastern special meeting. Accordingly, assuming a quorum is present at the Eastern special meeting, an abstention or a broker non-vote or other failure to vote will have no effect on the outcome of the Eastern Adjournment Proposal.

Proxies submitted by mail must be received by the close of business on Monday, January 8, 2024. Proxies submitted by telephone or through the Internet must be received by                    , Eastern Time, on                     .

Shares Held in Brokerage Accounts: If you hold your shares of Eastern common stock in street name (that is, you hold your shares of Eastern common stock through a broker, bank or other holder of record), your bank, broker or other holder of record will forward proxy materials and voting instructions that you must follow in order to vote your shares of Eastern common stock. You may receive more than one proxy card if your shares of Eastern common stock are registered in different names or are held in more than one account. A broker, bank or other holder of record will not be able to vote your shares at the Eastern special meeting without first receiving instructions from you on how to vote.

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Shares Held in the Eastern 401(k) Plan and ESOP. If you are a participant in Eastern’s 401(k) Plan or ESOP and indirectly hold shares of Eastern common stock through the Eastern 401(k) Plan or ESOP, the trustee or administrator votes shares of Eastern common stock held by the plan. However, you may direct the trustee or administrator on how to vote the shares of Eastern common stock allocated to your plan account. To do so, your voting instructions must be received by no later than                 . If you do not provide instructions by that date, the respective plan trustees or administrators will vote your shares in accordance with the terms of the respective plans. Please note that plan participants must provide voting instructions by the specified deadline (                ), which is earlier than the voting deadline for shareholders of record.

Additional information on attending the Eastern special meeting can be found under the section titled “The Special Meeting of Eastern Shareholders—Attending the Eastern Special Meeting” on page 51 of this joint proxy statement/prospectus.

c/o Corporate Secretary, Eastern Bankshares, Inc.

265 Franklin Street

Boston, Massachusetts 02110

Participants in the Eastern 401(k) Plan may revoke their instructions to the Eastern 401(k) Plan administrator with respect to voting of the shares of Eastern common stock held in their Eastern 401(k) Plan account by submitting to the Eastern 401(k) Plan administrator a signed instruction card bearing a later date, provided that such new instruction card must be received by the Eastern 401(k) Plan administrator on or prior to the last date for submission of such instructions with respect to the Eastern special meeting designated in the separate voting instructions provided by the Eastern 401(k) Plan administrator.

Participants in the Eastern ESOP may revoke their instructions to the Eastern ESOP trustee with respect to voting of the shares of Eastern common stock held in their Eastern ESOP account by submitting to the Eastern ESOP trustee a signed instruction card bearing a later date, provided that such new instruction card must be received by the Eastern ESOP trustee on or prior to the last date for submission of such instructions with respect to the Eastern special meeting designated in the separate voting instructions provided by the Eastern ESOP trustee.

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The Eastern proxy card will instruct the persons named on the Eastern proxy card to vote the Eastern shareholder’s shares of Eastern common stock at the Eastern special meeting as the shareholder directs. If an Eastern shareholder signs, dates and sends in an Eastern proxy card and does not indicate how the shareholder wishes to vote, the proxy will be voted “FOR” both of the Eastern special meeting proposals.

Eastern Bank Investor Relations

265 Franklin Street

Boston, Massachusetts 02110

(781) 598-7920

InvestorRelations@easternbank.com

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