First Interstate is a financial and bank holding company focused on community banking. Since its incorporation in Montana in 1971, First Interstate has grown both organically and through strategic acquisitions. First Interstate operates 147 banking offices, including detached drive-up facilities, in communities across six states: Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. Through its bank subsidiary, First Interstate Bank, First Interstate delivers a comprehensive range of banking products and services, including online and mobile banking, to individuals, businesses, municipalities, and others throughout its market areas. As of JuneSeptember 30, 2021, First Interstate had consolidated assets of $18.9$19.4 billion, deposits of $15.6$16.0 billion, loans held for investment of $9.8$9.6 billion, and total stockholders’ equity of $2.0 billion.

Great Western is a bank holding company incorporated in the state of Delaware and maintains its principal executive office in Sioux Falls, South Dakota. Great Western is a full-service regional bank holding company focused on relationship-based business banking through its wholly owned banking subsidiary, Great Western Bank. Great Western serves its customers through 175 branches in nine states: Arizona, Colorado, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota and South Dakota. Great Western provides a wide range of services, including commercial and consumer loan and depository services, private banking, brokerage, trust, investment advisory and other traditional banking services. At JuneSeptember 30, 2021, Great Western had total consolidated assets of $13.1$12.9 billion, total consolidated loans of $8.5$8.2 billion, total consolidated deposits of $11.5$11.3 billion, and total consolidated stockholders’ equity of $1.2 billion.

The terms and conditions of the merger are contained in the merger agreement, a copy of which is attached as Annex A to this joint proxy statement/prospectus. You are encouraged to read the merger agreement carefully and in its entirety, as it is the primary legal document that governs the merger.

Subject to the terms and conditions of the merger agreement, at the completion of the merger, Great Western will merge with and into First Interstate, with First Interstate as the surviving corporation. Following the completion of the merger, Great Western Bank will merge with and into First Interstate Bank, with First Interstate Bank as the surviving bank in the bank merger. Following the merger, Great Western common stock will be delisted from the NYSE and deregistered under the Exchange Act and will cease to be publicly traded.

In the merger, holders of Great Western common stock will receive 0.8425 shares of First Interstate Class A common stock for each share of Great Western common stock they hold immediately prior to the effective time. First Interstate will not issue any fractional shares of First Interstate Class A common stock in the merger. Holders of Great Western common stock who would otherwise be entitled to a fraction of a share of First Interstate Class A common stock in the merger will instead receive, for the fraction of a share, an amount in cash (rounded to the nearest cent) based on the First Interstate closing share value.

First Interstate Class A common stock is listed on the NASDAQ under the symbol “FIBK”, and Great Western common stock is listed on the NYSE under the symbol “GWB”. The following table shows the closing sale prices of First Interstate Class A common stock as reported on the NASDAQ on September 15, 2021 and Great Western common stock as reported on the NYSE on September 15, 2021, the last full trading day before the public announcement of the merger agreement, and on [   ],December 10, 2021, the last practicable trading day before the date of this joint proxy statement/prospectus. This table also shows the implied value of the merger consideration to be issued in exchange for each share of Great Western common stock, which was calculated by multiplying the closing price of First Interstate Class A common stock on those dates by the exchange ratio of 0.8425.

For more information on the exchange ratio, see the section entitled “The Merger—Terms of the Merger” beginning on page 6162 and the section entitled “The Transaction Agreements-Description of the Merger Agreement—Merger Consideration” beginning on page 120121.

At the effective time, each outstanding Great Western RSU and Great Western PSU will vest and be converted into the right to receive the merger consideration with respect to each share of Great Western common stock underlying such Great Western RSU or Great Western PSU as though such awards had been settled in shares of Great Western common stock immediately prior to the effective time, with the number of shares underlying Great Western PSUs determined assuming performance goals are satisfied at the greater of target and actual levels of performance as determined by the Great Western board of directors or a committee thereof prior to the effective time.

The merger has been structured to qualify as a reorganization for federal income tax purposes, and it is a condition to our respective obligations to complete the merger that First Interstate and Great Western each receive a legal opinion to the effect that the merger will so qualify. Assuming the receipt and accuracy of these opinions, holders of Great Western common stock generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of their Great Western common stock for First Interstate Class A common stock in the merger, except for any gain or loss that may result from the receipt of cash instead of a fractional share of First Interstate Class A common stock.

The First Interstate board of directors has determined that the merger, the merger agreement and the transactions contemplated by the merger agreement are advisable and in the best interests of First Interstate and its shareholders and has unanimously adopted and approved the merger agreement, the merger and the other transactions contemplated by the merger agreement. The First Interstate board of directors unanimously recommends that holders of First Interstate common stock vote “FOR” the First Interstate merger proposal, “FOR” the First Interstate authorized share count proposal, “FOR” the First Interstate staggered board proposal, and “FOR” the First Interstate adjournment proposal. For a more detailed discussion of the First Interstate board of directors’ recommendation, see the section entitled “The Merger—First Interstate’s Reasons for the Merger; Recommendation of the First Interstate Board of Directors” beginning on page 8485.

The Great Western board of directors has determined that the merger, the merger agreement and the transactions contemplated by the merger agreement are advisable, fair to and in the best interests of Great Western and its stockholders and has unanimously adopted and approved the merger agreement, the merger and the other transactions contemplated by the merger agreement. The Great Western board of directors unanimously recommends that holders of Great Western common stock vote “FOR” the Great Western merger proposal, “FOR” the Great Western compensation proposal and “FOR” the Great Western adjournment proposal. For a more detailed discussion of the Great Western board of directors’ recommendation, see the section entitled “The Merger—Great Western’s Reasons for the Merger; Recommendation of the Great Western Board of Directors” beginning on page 7273 of this joint proxy statement/prospectus.

In connection with the merger, Keefe, Bruyette & Woods, Inc. (“KBW”) delivered its opinion, dated September 15, 2021, to the First Interstate board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to First Interstate of the exchange ratio in the proposed merger. The full text of KBW’s opinion, which describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by KBW in preparing the opinion, is attached as Annex C to this joint proxy statement/prospectus. The opinion was for the information of, and was directed to, the First Interstate board of directors (in its capacity as such) in connection with its consideration of the financial terms of the merger. The opinion did not address the underlying business decision of First Interstate to engage in the merger or enter into the merger agreement or constitute a recommendation to the First Interstate board of directors in connection with the merger, and it does not constitute a recommendation to any holder of First Interstate Class A common stock or First Interstate Class B common stock or any shareholder or stockholder of any other entity as to how to vote in connection with the merger or any other matter.

On September 15, 2021, Barclays Capital, Inc. (“Barclays”) rendered its opinion to the First Interstate board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to First Interstate of the exchange ratio in the proposed merger. The full text of Barclays’ written opinion, dated September 15, 2021, is attached as Annex D to this joint proxy statement/prospectus. Barclays’ written opinion sets forth, among other things, the assumptions made, procedures followed, factors considered and limitations upon the review undertaken by Barclays in rendering its opinion. Barclays’ opinion was for the use and benefit of the First Interstate board of directors and was rendered to the First Interstate board of directors in connection with its consideration of the merger, addresses only the fairness, from a financial point of view, of the exchange ratio to be paid by First Interstate in the merger and does not constitute a recommendation to any shareholder of

Great Western retained Piper Sandler & Co. (“Piper Sandler”) to act as financial advisor to the Great Western board of directors in connection with Great Western’s consideration of a possible business combination.

At the September 15, 2021 meeting at which the Great Western board of directors considered the merger and the merger agreement, Piper Sandler delivered to the Great Western board of directors its oral opinion, which was subsequently confirmed in writing on September 15, 2021, to the effect that, as of such date, and based upon and subject to the factors and assumptions set forth therein, the exchange ratio was fair to the holders of Great Western common stock from a financial point of view. The full text of Piper Sandler’s opinion is attached as Annex E to this joint proxy statement/prospectus. The opinion outlines the procedures followed, assumptions made, matters considered and qualifications and limitations on the review undertaken by Piper Sandler in rendering its opinion. The description of the opinion set forth in this joint proxy statement/prospectus is qualified in its entirety by reference to the full text of the opinion. Holders of Great Western common stock are urged to read the entire opinion carefully in connection with their consideration of the merger and adoption of the merger agreement. Piper Sandler’s opinion was addressed to the Great Western board of directors (in its capacity as such) in connection with and for the purposes of its consideration of the merger and the merger agreement, and does not constitute a recommendation to any stockholder of Great Western as to how any such stockholder should vote at any meeting of stockholders called to consider and vote upon the adoption of the merger and the merger agreement.

Holders of First Interstate common stock are not entitled to appraisal or dissenters’ rights under the MBCA and holders of Great Western common stock are not entitled to appraisal or dissenters’ rights under the DGCL. For more information, see the section entitled “The Merger—Appraisal or Dissenters’ Rights in the Merger” beginning on page 118119.

In considering the recommendation of the First Interstate board of directors to vote for the First Interstate merger proposal, the First Interstate authorized share count proposal, the First Interstate staggered board proposal, and the First Interstate adjournment proposal, holders of First Interstate common stock should be aware that the directors and executive officers of First Interstate may have interests in the merger that are different from, or in addition to, the interests of holders of First Interstate common stock generally. The First Interstate board of directors was aware of these interests and considered them, among other matters, in making its recommendation that First Interstate shareholders vote to approve the First Interstate merger proposal, the First Interstate authorized share count proposal, the First Interstate staggered board proposal and the First Interstate adjournment proposal.

The First Interstate board of directors was aware of and considered these respective interests when deciding to adopt and approve the merger agreement and the other transaction agreements. For more information, see the section entitled “The Merger—Interests of First Interstate’s Directors and Executive Officers in the Merger” beginning on page 109110.

In considering the recommendation of the Great Western’s board of directors to vote for the Great Western merger proposal, the Great Western compensation proposal and the Great Western adjournment proposal, holders of Great Western common stock should be aware that the directors and executive officers of Great Western may have interests in the merger that are different from, or in addition to, the interests of holders of Great Western common stock generally. The Great Western board of directors was aware of these interests and considered them, among other matters, in making its recommendation that Great Western shareholders vote to approve the Great Western merger proposal, the Great Western compensation proposal and the Great Western adjournment proposal.

The Great Western board of directors was aware of and considered these respective interests when deciding to adopt and approve the merger agreement. For more information, see the section entitled “The Merger—Interests of Great Western’s Directors and Executive Officers in the Merger” beginning on page 110111.

In connection with the merger, First Interstate’s articles of incorporation will be amended to increase the number of authorized shares of First Interstate Class A common stock from one hundred million (100,000,000) shares to one hundred fifty million (150,000,000) shares. Further, First Interstate’s articles of incorporation will be amended to make certain technical changes, which are intended to incorporate provisions that currently exist in First Interstate’s bylaws, relating to the classification of the First Interstate board of directors into three classes, with directors in each class serving staggered three-year terms. A copy of the First Interstate articles amendment is attached to this joint proxy statement/prospectus as Annex B.

Except as described above, First Interstate’s articles of incorporation as in effect immediately prior to the effective time, as amended as described above, will be the articles of incorporation of the surviving corporation, until thereafter amended in accordance with applicable law.

Prior to the closing of the merger (the “closing”), the First Interstate board of directors will take all actions necessary to cause the bylaws of First Interstate to be amended as set forth in Exhibit B to the merger agreement (such amendment, the “First Interstate bylaw amendment”), and as so amended, effective upon the completion of the merger, the bylaws of First Interstate will be the bylaws of the surviving corporation, until thereafter amended as provided therein or in accordance with applicable law. The First Interstate bylaw amendment provides that, from and after the effective time and until the date of the conversion, neither First Interstate nor the First Interstate board of directors may take any action that would prevent the conversion, and the provision in the bylaw amendment implementing such conversion arrangement may not be amended or repealed by the board of directors of the surviving corporation except by an affirmative vote of at least seventy-five percent (75%) of the entire board of directors of the surviving corporation (including at least one legacy Great Western director (as defined below)).

The board of directors of the surviving corporation as of the effective time will have sixteen (16) members, consisting of:

Prior to the effective time, the parties will cooperate in good faith (coordinating through their respective Chairs) to mutually agree on the selection of the Great Western directors who will join the board of directors of the surviving corporation, their respective classes and their respective committee appointments, taking into account relevant considerations including skill sets, experience, diversity and inclusion, and the needs of the board of directors of the surviving corporation; provided, that (i) the legacy Great Western directors will be apportioned among the three (3) classes of the board of directors of the surviving corporation as nearly evenly as is possible, (ii) the Chair of the Great Western board of directors as of immediately prior to the effective time will be appointed to the Executive Committee of the board of directors of the surviving corporation effective as of the effective time and (iii) the legacy Great Western directors will be eligible and given due consideration for committee service to the same extent as the legacy First Interstate directors, and each legacy Great Western director will be appointed to at least two (2) standing committees of the board of directors of the surviving corporation effective as of the effective time.

In addition, pursuant to the merger agreement, the surviving corporation, the board of directors of the surviving corporation and the Governance and Nominating Committee of the board of directors of the surviving corporation will take all actions necessary to nominate the legacy Great Western directors for reelection to the board of directors of the surviving corporation at the first annual meeting of shareholders of the surviving corporation following the effective time, and thereafter (provided such directors continue to meet the director qualification and eligibility criteria of the Governance and Nominating Committee of the board of directors of the surviving corporation) any legacy Great Western director whose class term expires in fewer than three (3) years from the date on which closing of the merger occurs (the “closing date”) will be nominated for reelection to the board of directors of the surviving corporation upon the expiration of his or her term, so that each legacy Great Western director will serve as a member of the board of directors of the surviving corporation for a minimum of three (3) full years from the closing date.

In addition, pursuant to the merger agreement, First Interstate and Great Western have agreed to cooperate in good faith to develop, and make recommendations for approval by the board of directors of the surviving corporation with respect to, any advisable changes to the corporate governance guidelines and board committee charters of the surviving corporation to comply with applicable law and the listing requirements and corporate governance rules of NASDAQ in anticipation of the surviving corporation no longer qualifying as a “controlled company” under the NASDAQ Marketplace LLC Rules (the “NASDAQ rules”).

Subject to the terms of the merger agreement, First Interstate and Great Western have agreed to cooperate with each other and use reasonable best efforts to promptly prepare and file all documentation to obtain as promptly as practicable all permits, consents, orders, approvals, waivers, non-objections and authorizations of all third parties and governmental entities which are necessary or advisable to consummate the transactions contemplated by the merger agreement (including the merger and the bank merger), and to comply with the terms and conditions of all such permits, consents, orders, approvals, waivers, non-objections and authorizations of all such governmental entities. These approvals include the approval of the Board of Governors of the Federal Reserve System (the “Federal Reserve Board”), the Montana Division of Banking and Financial Institutions (“MDOB”) and the Division of Banking of the South Dakota Department of Labor and Regulation (“SDDB”).

Although neither First Interstate nor Great Western knows of any reason why it cannot obtain these regulatory approvals in a timely manner, First Interstate and Great Western cannot be certain when or if they will be obtained, or that the granting of these regulatory approvals will not involve the imposition of conditions on the completion of the merger or the bank merger.

First Interstate and Great Western expect the merger to close in the first quarter of 2022. However, neither First Interstate nor Great Western can predict the actual date on which the merger will be completed, or if the merger will be completed at all, because completion is subject to conditions and factors outside the control of both companies. First Interstate and Great Western must first obtain the approval of holders of First Interstate common stock and holders of Great Western common stock for the merger, as well as obtain necessary regulatory approvals and satisfy certain other closing conditions.

As more fully described in this joint proxy statement/prospectus and in the merger agreement, the completion of the merger depends on a number of conditions being satisfied or, where legally permissible, waived. These conditions include:

The merger agreement may be terminated at any time prior to the completion of the merger, whether before or after the receipt of the requisite First Interstate vote or the requisite Great Western vote (except as indicated below), in the following circumstances:

If the merger agreement is terminated by either First Interstate or Great Western under certain circumstances, including circumstances involving alternative acquisition proposals and changes in the recommendation by First Interstate or Great Western or their respective boards, First Interstate may be required to pay a termination fee of $105 million to Great Western and Great Western may be required to pay a termination fee of $70 million to First Interstate.

Concurrently with the execution and delivery of the merger agreement, the Scott Family shareholders entered into the support agreement pursuant to which, among other things, each Scott Family shareholder has agreed, subject to the terms of the support agreement, to (i) vote the shares of First Interstate common stock owned by it in favor of the approval and adoption of the merger agreement and the First Interstate articles amendment, and against any competing transaction and (ii) not transfer its shares of First Interstate common stock prior to the First Interstate special meeting, with certain limited exceptions. The support agreement will terminate upon termination of the merger agreement and certain other specified events, including, among others, if the First Interstate board effects a recommendation change under the merger agreement and a majority of the independent directors of First Interstate approve such recommendation change. Each of First Interstate and Great Western is an express third party beneficiary of the support agreement and has the right to directly enforce the obligations of the parties thereto. As of [   ],December 13, 2021, the record date for the First Interstate special meeting, the Scott family shareholders collectively owned [   ]%53.1% of the voting power represented by issued and outstanding shares of First Interstate common stock.

Concurrently with the execution and delivery of the merger agreement, the Scott Family shareholders entered into a stockholders’ agreement with First Interstate (the “stockholders’ agreement”), which will become effective as of the closing of the merger.

Under the stockholders’ agreement, if the merger is consummated in accordance with the terms of the merger agreement, then First Interstate will pay to the Scott Family shareholders all reasonable and documented out-of-pocket expenses incurred by the Scott Family shareholders in connection therewith, up to a maximum of $8.5 million.

The stockholders’ agreement also provides the Stockholders (i) up to two “demand” registrations in the aggregate in any 12-month period in the case of a marketed underwritten offering, (ii) up to four underwritten block trades in the aggregate in any 12-month period and (iii) customary “piggyback” registration rights. The stockholders’ agreement also provides that First Interstate will indemnify the Stockholders against certain liabilities that may arise under the Securities Act.

Concurrently with the execution and delivery of the merger agreement, the Scott Family shareholders entered into a letter agreement with First Interstate (the “letter agreement”). Under the letter agreement, First Interstate agrees that, in the event the merger agreement is terminated prior to the closing in a circumstance in which First Interstate receives payment of the termination fee from Great Western, First Interstate will pay to the Scott Family shareholders all reasonable and documented out-of-pocket expenses incurred by the Scott Family shareholders in connection therewith, up to a maximum of $3.5 million.

First Interstate and Great Western each prepare their respective financial statements in accordance with accounting principles generally accepted in the United States (“GAAP”). The merger will be accounted for using the acquisition method of accounting, and First Interstate will be treated as the accounting acquirer for financial reporting purposes.

The rights of holders of Great Western common stock are governed by Delaware law and by the certificate of incorporation and bylaws of Great Western. In the merger, holders of Great Western common stock will become holders of common stock of the surviving corporation, and their rights will be governed by Montana law and the articles of incorporation of First Interstate as amended by the First Interstate articles amendment and the bylaws of First Interstate as amended by the First Interstate bylaw amendment. Holders of Great Western common stock will have different rights once they become holders of common stock of the surviving corporation due to differences between the Great Western governing documents and Delaware law, on the one hand, and the First Interstate governing documents and Montana law, on the other hand. These differences are described in more detail under the section entitled “Comparison of Shareholders’ Rights” beginning on page 145146.

The shares of First Interstate Class A common stock to be issued in the merger will be listed for trading on the NASDAQ. Following the merger, shares of First Interstate Class A common stock will continue to be listed on the NASDAQ. In addition, following the merger, Great Western common stock will be delisted from the NYSE and deregistered under the Exchange Act.

The First Interstate special meeting will be held at First Interstate Bank Great West Center, 1800 Sixth Avenue North, Billings, MT 59101, on [   ],January 19, 2022, at [   ], local3:30 p.m., mountain time. At the First Interstate special meeting, holders of First Interstate common stock will be asked to consider and vote on the following proposals:

You may vote at the First Interstate special meeting if you owned shares of First Interstate common stock at the close of business on [   ].December 13, 2021. On that date, there were [   ]41,686,490 shares of First Interstate Class A common stock and [   ]20,514,347 shares of First Interstate Class B common stock outstanding, of which approximately [ ]%1.1% of the First Interstate Class A common stock and [ ]%31.7% of the First Interstate Class B common stock were owned and entitled to be voted by First Interstate directors and executive officers and their affiliates. We currently expect that First Interstate’s directors and executive officers will vote their shares in favor of the First Interstate merger proposal and the other proposals to be considered at the First Interstate special meeting, although none of them has entered into any agreements obligating them to do so other than the Scott Family shareholders who are parties to the support agreement. As of [   ],December 13, 2021, the record date for the First Interstate special meeting, the Scott family shareholders collectively owned approximately [   ]%53.1% of the voting power represented by issued and outstanding shares of First Interstate common stock.

The Great Western special meeting will be held virtually on [   ],January 19, 2022, at [   ], local4:30 p.m., central time. At the Great Western special meeting, holders of Great Western common stock will be asked to consider and vote on the following proposals:

You may vote at the Great Western special meeting if you owned shares of Great Western common stock at the close of business on [   ].December 13, 2021. On that date, there were [   ]55,199,193 shares of Great Western common stock outstanding, approximately [   ]less than one percent ([   ]%(1.0%) of which were owned and entitled to be voted by Great Western directors and executive officers and their affiliates. We currently expect that Great Western’s directors and executive officers will vote their shares in favor of the Great Western merger proposal and the other proposals to be considered at the Great Western special meeting, although none of them has entered into any agreements obligating them to do so.

The Great Western merger proposal will be approved if the holders of a majority of the outstanding shares of Great Western common stock entitled to vote on such proposal vote in favor of such proposal. The Great Western compensation proposal and the Great Western adjournment proposal will each be approved if the holders of a majority of the shares of Great Western common stock present or represented by proxy at the Great Western special meeting and entitled to vote on the subject matter vote in favor of such proposal. If you mark “ABSTAIN” on your proxy, fail to submit a proxy or vote at the Great Western special meeting or fail to instruct your bank, broker, trustee or other nominee how to vote with respect to the Great Western merger proposal, it will have the same effect as a vote “AGAINST” the Great Western merger proposal. If you mark “ABSTAIN” on your proxy with respect to the Great Western compensation proposal or the Great Western adjournment proposal, it will have the same effect as a vote “AGAINST” such proposal. If you fail to submit a proxy or vote at the Great Western special meeting or fail to instruct your bank, broker, trustee or other nominee how to vote with respect to the Great Western compensation proposal or the Great Western adjournment proposal, it will have no effect on such proposal.

In evaluating the merger agreement and the merger, including the issuance of shares of First Interstate Class A common stock in the merger, you should carefully read this joint proxy statement/prospectus and give special consideration to the factors discussed in the section entitled “Risk Factors” beginning on page 3536 and in First Interstate’s Annual Report on Form 10-K for the year ended December 31, 2020 and Great Western’s Annual Report on Form 10-K for the year ended September 30, 20202021 and in other documents incorporated by reference into this joint proxy statement/prospectus. Please see the section entitled “Where You Can Find More Information” beginning on page 161162 of this joint proxy statement/prospectus for the location of information incorporated by reference into this joint proxy statement/prospectus.

The following unaudited pro forma combined condensed consolidated financial information is based on the separate historical financial statements of First Interstate and Great Western after giving effect to the merger and the issuance of First Interstate Class A common stock in connection therewith, and the assumptions and adjustments described in the accompanying notes to the unaudited pro forma combined condensed financial information. The unaudited pro forma combined condensed consolidated statement of income for the year ended December 31, 2020 combines the historical consolidated statements of income of First Interstate and Great Western, giving effect to the merger as if it had been completed on January 1, 2020. The unaudited pro forma combined condensed consolidated statement of income for the period ended JuneSeptember 30, 2021 combines the historical consolidated statements of income of First Interstate and Great Western, giving effect to the merger as if it had been completed on January 1, 2021. The accompanying unaudited pro forma combined condensed consolidated balance sheet as of JuneSeptember 30, 2021 combines the historical consolidated balance sheets of First Interstate and Great Western, giving effect to the merger as if it had been completed on JuneSeptember 30, 2021.

The following unaudited pro forma combined condensed consolidated financial information and related notes are based on and should be read in conjunction with (i) the historical audited consolidated financial statements of First Interstate and the related notes included in First Interstate’s Annual Report on Form 10-K for the year ended December 31, 2020 and unaudited consolidated financial statements of First Interstate and the related notes included in First Interstate’s Quarterly Report on Form 10-Q for the period ended JuneSeptember 30, 2021, each of which is incorporated into this joint proxy statement/prospectus by reference, and (ii) the historical audited consolidated financial statements of Great Western and the related notes included in Great Western’s Annual Report on Form 10-K for the yearyears ended September 30, 2020 and unaudited consolidated financial statementsSeptember 30, 2021, which, in the case of Great Westernthe Annual Report on Form 10-K for the year ended September 30, 2021, is incorporated by reference herein, and the related notes included in Great Western’s Quarterly Report on Form 10-Q for the periodperiods ended December 31, 2020, March 31, 2021 and June 30, 2021, each of which isare not incorporated into this joint proxy statement/prospectus by reference.

The historical consolidated financial information has been adjusted in the unaudited pro forma combined condensed consolidated financial information to give effect to the pro forma events that are (i) directly related to the merger, (ii) factually supportable and (iii) with respect to the unaudited pro forma combined condensed statement of income, expected to have a continuing effect on the results of the combined company. The unaudited pro forma combined condensed consolidated financial information contained herein does not reflect the costs of any integration activities or benefits that may result from the realization of future cost savings from operating efficiencies, or any other synergies that may result from the merger. The following unaudited pro forma combined condensed consolidated financial information gives effect to the merger and includes adjustments for the following:

Future results may differ materially from the results reflected because of various factors, including those discussed in the section entitled “Risk Factors” beginning on page 3536 and appearing under the caption “Risk Factors” in First Interstate’s and Great Western’s most recently filed Annual Reports on Form 10-K and in any subsequently filed Quarterly Reports on Form 10-Q or Current Reports on Form 8-K, which are incorporated by reference in this joint proxy statement/prospectus, and the factors discussed in the section entitled “Cautionary Statement Regarding Forward-Looking Statements” beginning on page 3334. Among other factors, the actual amounts recorded as of the completion of the merger may differ materially from the information presented in these unaudited pro forma combined condensed consolidated financial statements as a result of:

The risk of such variance is particularly significant with respect to the preliminary purchase price allocation, because such allocation is based, in large part, on the closing price per share of First Interstate Class A common stock as of the closing date. The pro forma adjustments are preliminary estimates for intangible assets and certain financial assets and financial liabilities. The purchase price allocation and related adjustments reflected in these unaudited pro forma combined condensed consolidated financial statements are preliminary and subject to revision based on final determination of fair value. The preliminary purchase price allocation reflected in the unaudited pro forma combined condensed consolidated financial information assumes a closing price per share of First Interstate Class A common stock of $42.32,$40.26, the closing price of First Interstate Class A common stock on September 15, 2021, or the date of the merger agreement.30, 2021. The financial markets have recently experienced extreme volatility, due in large part to the coronavirus pandemic and its widespread economic impacts. Continued financial market volatility, and its effect on the trading prices of First Interstate Class A common stock and Great Western common stock, will largely depend on future developments, which First Interstate and Great Western cannot accurately predict or control, including new information which may emerge, the effectiveness or ineffectiveness of governmental and private actions taken to contain or treat the coronavirus pandemic, and reactions by companies, consumers, investors, governmental entities and financial markets to such actions. Particularly given this volatility and uncertainty, the unaudited pro forma combined condensed consolidated financial information included in this joint proxy statement/prospectus may not be indicative of and does not purport to represent the combined company’s actual financial condition or results of operations as of the closing date or any future or other date or period.

The following unaudited pro forma combined condensed consolidated financial information and related notes are being provided for illustrative purposes only and do not purport to represent what the combined company’s actual results of operations or financial position would have been had the merger been completed on the dates indicated, nor are they necessarily indicative of the combined company’s future results of operations or financial position for any future period. The preparation of the unaudited pro forma combined condensed consolidated financial statements and related adjustments required management to make certain assumptions and estimates. The unaudited pro forma combined condensed consolidated financial statements should be read together with:

The following unaudited pro forma combined condensed consolidated balance sheet gives effect to the acquisition by First Interstate using the acquisition method of accounting assuming the acquisition was consummated on JuneSeptember 30, 2021.

The following unaudited pro forma combined condensed consolidated statements of income for the year ended December 31, 2020 gives effect to First Interstate’s acquisition of Great Western using the acquisition method of accounting assuming the acquisition was consummated on January 1, 2020.