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Registration No. 333-     

As filed with the Securities and Exchange Commission on February 20, 2014

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-4

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933

First Interstate BancSystem, Inc.

(Exact name of registrant as specified in its charter)

401 North 31st Street

Billings, Montana 59101

(406) 255-5390

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

Kevin P. Riley

Executive Vice President and Chief Financial Officer

401 North 31st Street

Billings, Montana 59101

(406) 255-5390

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

Approximate date of commencement of proposed sale of the securities to the public:

As soon as practicable after the effective date of this registration statement.

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: o

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. o

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. o

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

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

CALCULATION OF REGISTRATION FEE

(1) Represents the maximum number of shares of Class A Common Stock, no par value per share, of First Interstate BancSystem, Inc. that may be issued in connection with the proposed merger to which this registration statement relates.

(2) Estimated solely for the purpose of calculating the amount of the registration fee required by Section 6(b) of the Securities Act of 1933, as amended, and computed pursuant to Rule 457(f), based upon the product of (a) 5,660,748, the maximum number of shares of common stock of Mountain West Financial Corp. to be exchanged or cancelled in the merger and (b) $11.77, the book value per share of common stock of Mountain West Financial Corp. as of the most reasonable practicable date, minus the amount of cash to be paid, per share by First Interstate BancSystem, Inc. in the merger.

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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Information contained herein is subject to completion or amendment. A registration statement relating to the shares of First Interstate BancSystem, Inc. Class A Common Stock to be issued in the merger has been filed with the Securities and Exchange Commission. These securities may not be sold nor may offers to buy be accepted prior to the time the registration statement becomes effective. This proxy statement/prospectus shall not constitute an offer to sell or the solicitation of an offer to buy these securities nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale is not permitted or would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.

PRELIMINARY PROXY STATEMENT/PROSPECTUS
DATED FEBRUARY 20, 2014, SUBJECT TO COMPLETION

MOUNTAIN WEST FINANCIAL CORP.

To the Stockholders of Mountain West Financial Corp.:

We are furnishing this proxy statement/prospectus to the holders of common stock of Mountain West Financial Corp., which we refer to as Mountain West, in connection with the solicitation of proxies for a meeting of the stockholders of Mountain West concerning the proposals described in this document.  On February 10, 2014, First Interstate BancSystem, Inc., which we refer to as First Interstate, entered into a merger agreement to acquire Mountain West, in a transaction in which the consideration will be paid partly in stock and partly in cash. If the merger agreement is approved and the merger is subsequently completed, Mountain West will merge with and into First Interstate, with First Interstate as the surviving entity.  The boards of Mountain West and First Interstate have each unanimously approved the merger agreement.  Furthermore, all current directors and certain senior executive officers of Mountain West, who held approximately         % of the issued and outstanding shares of Mountain West common stock on the record date, have signed agreements to support the merger and vote in favor of it at the Mountain West stockholder meeting.

Under the merger agreement, at the effective time of the merger, each issued and outstanding share of Mountain West common stock will be cancelled and converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash or, if a Mountain West stockholder properly elects and subject to the limitations summarized below, an amount in all cash or all stock roughly equal in value to the above described combination of stock and cash merger consideration. Based on the closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market on                    , 2014, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on such date, the aggregate value of the consideration that would be expected to be delivered to Mountain West stockholders in the merger would be approximately $      million, of which $      million would be paid in cash and $      million would be represented by                     shares of First Interstate Class A Common Stock.  The actual value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the First Interstate Class A Common Stock. You may obtain current stock price quotations for First Interstate Class A Common Stock on the NASDAQ Global Select Market under the symbol “FIBK.”  Mountain West common stock is not listed on a national securities exchange, and therefore current Mountain West stock price quotations are not available.

As an accommodation to Mountain West stockholders, First Interstate has agreed to allow them to elect to receive their individual merger consideration in either all cash or all First Interstate Class A Common Stock, subject to the limitations discussed below and in lieu of the stock and cash combination merger consideration.  Electing Mountain West stockholders may not ultimately receive in the merger only cash or only stock, however, because the merger agreement provides that First Interstate is only required to issue that number of shares of First Interstate Class A Common Stock that would have been issued in the merger if all Mountain West shares were converted at the specified stock and cash combination merger consideration, which we refer to as the target share number.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock at the target share number, then the Mountain West shares shall be converted in accordance with the elections.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received

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would result in the issuance of shares of First Interstate Class A Common Stock in excess of the target share number, then the number of shares of First Interstate Class A Common Stock to be delivered to the Mountain West stockholders making the all stock elections will be reduced proportionately and cash in an equivalent value will be paid to those stockholders.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of fewer shares of First Interstate Class A Common Stock than the target share number, then the amount of cash to be delivered to the Mountain West stockholders making the all cash elections will be reduced proportionately and shares of First Interstate Class A Common Stock having an equivalent value will be issued to those stockholders.

No fractional shares of First Interstate Class A Common Stock will be issued in the merger.  A Mountain West stockholder will receive cash in lieu of any fractional shares of First Interstate Class A Common Stock such holder would otherwise have been entitled to receive in the merger.

Mountain West will hold a special meeting of stockholders to consider the proposed merger and related matters. First Interstate and Mountain West cannot complete the proposed merger unless Mountain West’s stockholders vote to approve the merger agreement and the merger. This letter and the accompanying documents are being provided by the Mountain West board of directors to solicit your proxy to vote for approval of the merger agreement and the merger at a special meeting of Mountain West stockholders. The vote of the stockholders of First Interstate is not required to approve the merger agreement or merger.  This letter and the accompanying documents are also being delivered to Mountain West stockholders as First Interstate’s prospectus for its offering of First Interstate Class A Common Stock in the merger.

We expect the merger to be generally tax free to Mountain West stockholders for United States federal income tax purposes, except with respect to the taxes attributable to the cash to be received by Mountain West stockholders.  Mountain West stockholders are urged to consult your own tax advisors to determine the extent to which your receipt of the merger consideration may be taxable to you.

Your vote is very important. To ensure your representation at the Mountain West special meeting, please complete and return the enclosed proxy card or submit your proxy through the Internet. Whether or not you expect to attend the Mountain West special meeting, please promptly vote. Submitting a proxy now will not prevent you from being able to vote in person at the Mountain West special meeting. The Mountain West board of directors has unanimously approved the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the approval of the merger agreement and the merger and “FOR” the adjournment of the special meeting if necessary or appropriate.

The accompanying document provides you with additional detailed information about the proposed merger and the merger agreement. It also contains or references information about Mountain West and First Interstate and related matters. You are encouraged to read the accompanying document carefully, including the information incorporated in the document by reference. In particular, you should read the “RISK FACTORS” section beginning on page 19 for a discussion of the risks you should consider in evaluating the proposed merger and how it will affect you.

Sincerely,

Richard G. Anderson

Chairman of the Board

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the merger agreement or merger, the issuance of the First Interstate Class A Common Stock in connection with the merger or the other transactions described in this document, or passed upon the adequacy or accuracy of the disclosure in this letter or the accompanying document. Any representation to the contrary is a criminal offense.

The securities to be issued in connection with 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 any other governmental agency.

This letter and the accompanying documents are dated                    , 2014, and are first being mailed to stockholders of Mountain West on or about                          , 2014.

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MOUNTAIN WEST FINANCIAL CORP.
1225 Cedar Street

Helena, MT 59601

NOTICE OF THE SPECIAL MEETING OF STOCKHOLDERS
TO BE HELD ON                              , 2014

NOTICE IS HEREBY GIVEN that a special meeting of the stockholders of Mountain West Financial Corp., which we refer to as Mountain West, will be held at the Holiday Inn — Downtown, 22 N. Last Chance Gulch St., Helena, MT 59601, at 10:00 a.m., Mountain Time, on                           , 2014, for the following purposes:

1. To approve the Agreement and Plan of Merger, which we refer to as the merger agreement, dated as of February 10, 2014, by and between First Interstate BancSystem, Inc. and Mountain West, as such merger agreement may be amended from time to time, a copy of which is attached as Appendix A, and the merger to be effected pursuant thereto, which we refer to collectively as the Mountain West Merger proposal; and

2. To approve one or more adjournments of the Mountain West special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the Mountain West Merger proposal, which we refer to as the Mountain West Adjournment proposal.

Mountain West will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The Mountain West Merger proposal is described in more detail in this document, which you should read carefully in its entirety before you vote. A copy of the merger agreement is attached as Appendix A to this document.

The Mountain West board of directors has set                         , 2014 as the record date for the Mountain West special meeting. Only holders of record of Mountain West common stock at the close of business on                           , 2014 will be entitled to notice of and to vote at the Mountain West special meeting and any adjournments or postponements thereof. Any stockholder entitled to attend and vote at the Mountain West special meeting is entitled to appoint a proxy to attend and vote on such stockholder’s behalf. Such proxy need not be a holder of Mountain West common stock.

Your vote is very important. To ensure your representation at the Mountain West special meeting, please complete and return the enclosed proxy card or submit your proxy through the Internet. Please promptly vote whether or not you expect to attend the Mountain West special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the Mountain West special meeting.

The Mountain West board of directors has unanimously approved the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

By order of the Board of Directors,

Richard G. Anderson
Chairman of the Board

Helena, Montana

, 2014

PLEASE VOTE YOUR SHARES OF MOUNTAIN WEST COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL MOUNTAIN WEST INVESTOR RELATIONS AT 406-449-2265.

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ABOUT THIS DOCUMENT

This document, which forms part of a registration statement on Form S-4 filed with the Securities and Exchange Commission by First Interstate (File No. 333-               ), constitutes a prospectus of First Interstate under Section 5 of the Securities Act of 1933, as amended, which is referred to herein as the Securities Act, with respect to the shares of First Interstate Class A Common Stock to be issued in the merger contemplated by the merger agreement and the transactions contemplated thereby.

This document also constitutes a notice of meeting and a proxy statement with respect to Mountain West’s special meeting, at which Mountain West stockholders will be asked to consider and vote upon certain proposals, including a proposal to approve the merger and the merger agreement and the transactions contemplated thereby.

WHERE YOU CAN FIND MORE INFORMATION

First Interstate files annual, quarterly and current reports, proxy statements and other business and financial information with the Securities and Exchange Commission, which referred to herein as the SEC. You may read and copy any materials that First Interstate files with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549, at prescribed rates. Please call the SEC at (800) SEC-0330 ((800) 732-0330) for further information on the public reference room. In addition, First Interstate files its reports and other business and financial information with the SEC electronically, and the SEC maintains a website located at http://www.sec.gov containing this information. You are also able to obtain these documents, free of charge, from First Interstate at www.FIBK.com under the tab “SEC Filings.” The information contained on or accessible from First Interstate’s website does not constitute a part of this document and is not incorporated by reference herein.

First Interstate has filed a registration statement on Form S-4 of which this document forms a part. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits at the address set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates by reference documents that First Interstate has previously filed and may hereafter file with the SEC. They contain important information about First Interstate and its financial condition. For further information, please see the section entitled “INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE” beginning on page 79.

First Interstate will provide to each person to whom a copy of this prospectus is delivered, a copy of any or all of the information that it has incorporated by reference into this document (excluding exhibits to such documents unless such exhibits are specifically incorporated by reference). First Interstate will provide this information upon written or oral request at no cost to the requester. You may request this information by contacting First Interstate’s corporate headquarters at the following address: 401 North 31st Street, Billings, Montana 59101, Attention: Amy Anderson, or by calling (406) 255-5390.

To obtain timely delivery of these documents, you must request the information no later than                   , 2014 in order to receive them before Mountain West’s special meeting of stockholders.

First Interstate Class A Common Stock is traded on the NASDAQ Global Select Market under the symbol “FIBK.”

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETING

WHAT IS THE MERGER?

First Interstate and Mountain West have entered into an Agreement and Plan of Merger, which is referred to herein as the merger agreement, pursuant to which Mountain West will merge with and into First Interstate with First Interstate continuing as the surviving corporation in a transaction that is referred to in this proxy statement/prospectus as the merger. A copy of the merger agreement is attached as Appendix A to this document. If the merger is consummated, First Interstate and Mountain West expect that within six months Mountain West Bank, National Association, a wholly owned subsidiary of Mountain West and which is referred to in this proxy statement/prospectus as Mountain West Bank, will merge with and into First Interstate Bank, which is a wholly owned subsidiary of First Interstate.  The merger of Mountain West Bank with and into First Interstate Bank is referred in this proxy statement/prospectus as the bank merger. In order for Mountain West to complete the transaction with First Interstate, the approval of Mountain West’s stockholders is needed as are approvals by the bank regulators of First Interstate, Mountain West, First Interstate Bank, and Mountain West Bank.

WHY AM I RECEIVING THIS PROXY STATEMENT/PROSPECTUS?

Mountain West is sending these materials to its stockholders to help them decide how to vote their shares of Mountain West common stock with respect to the merger and other matters to be considered at the special meeting. More specifically, the merger cannot be completed unless Mountain West stockholders approve the merger agreement. Mountain West is holding a special meeting of its stockholders to vote on the proposals necessary to complete the merger. Information about the special meeting, the merger and the other business to be considered by Mountain West’s stockholders at the special meeting is contained in this document.

In addition, this document constitutes a prospectus of First Interstate. It is a prospectus because First Interstate, in connection with the merger, is offering shares of its Class A Common Stock, in addition to cash consideration, in exchange for outstanding shares of Mountain West common stock in the merger. The shares being offered by this document are required to be registered with the SEC prior to their issuance.

WHAT WILL MOUNTAIN WEST STOCKHOLDERS RECEIVE IN THE MERGER?

Under the merger agreement, First Interstate expects to issue approximately                      shares of its Class A Common Stock and to pay approximately $      million in cash for all of the outstanding shares of common stock of Mountain West, based on the                     shares of Mountain West common stock and options to purchase shares of Mountain West common stock outstanding as of                      , 2014, which is referred to in this proxy statement/prospectus as the record date.

WHAT WILL EACH MOUNTAIN WEST STOCKHOLDER RECEIVE IN THE MERGER?

Each Mountain West stockholder will receive a combination of cash and stock consisting of 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash per share of Mountain West common stock; provided that, if a stockholder chooses, the stockholder may elect to receive one of the following forms of merger consideration instead of such cash and stock combination

·                       all cash, in an amount per share of Mountain West common stock equal, subject to proration and adjustment as described herein, to (i) $7.125 plus (ii) the product of (x) 0.2552 and (y) the average closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market during the twenty (20) business days immediately preceding the fifth day prior to the closing date of the merger, which is referred to herein as the average closing price; or

·                       all First Interstate Class A Common Stock, in an amount of shares of First Interstate Class A Common Stock per share of Mountain West common stock equal, subject to proration and adjustment as described herein, to (i) 0.2552 plus (ii) the quotient of (x) $7.125 divided by (y) the average closing price.

All elections are subject to the election, proration and allocation procedures and limitations described in this proxy statement/prospectus.  Due to these limitations, Mountain West stockholders electing all cash or all stock will be subject to a pro rata adjustment as may be necessary, because the merger agreement provides that First Interstate is only required to issue that number of shares of First Interstate Class A Common Stock that would have been issued in

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the merger if all Mountain West shares were converted at the specified stock and cash combination merger consideration (referred to herein as the target share number).

HOW AND WHEN DOES A MOUNTAIN WEST STOCKHOLDER ELECT THE FORM OF CONSIDERATION HE OR SHE PREFERS TO RECEIVE?

If a Mountain West stockholder desires to make an all cash or all stock election (rather than receive the established merger consideration consisting of a mix of cash and stock, for which no election need be made), a Mountain West stockholder must submit an election statement (which accompanies this proxy statement/prospectus) to the exchange agent before 5:00 p.m., Mountain Time, on the date that is five business days prior to the closing date of the merger (or such other time and date as First Interstate and Mountain West may mutually agree upon). This date is referred to as the election deadline. Election choices and election procedures are described under the caption entitled “THE MERGER.”

NOTE: The actual election deadline is not currently known. First Interstate and Mountain West will issue a press release announcing the date of the election deadline at least five business days prior to the election deadline. Additionally, First Interstate and Mountain West will post the date of the election deadline on their respective web sites, also at least five business days before that deadline.

MAY A MOUNTAIN WEST STOCKHOLDER CHANGE HIS OR HER ELECTION ONCE IT HAS BEEN SUBMITTED?

Yes. An election may be revoked or changed so long as the new election is received by the exchange agent at or prior to the election deadline. The exchange agent has reasonable discretion to determine whether any election, revocation or change has been timely made.

WHAT HAPPENS IF AN ELECTION IS NOT MADE PRIOR TO THE ELECTION DEADLINE?

If a Mountain West stockholder chooses not to submit an election statement to the exchange agent prior to the election deadline, then that holder will receive, for each share of Mountain West common stock owned by such holder, 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash.

WILL FRACTIONAL SHARES OF FIRST INTERSTATE CLASS A COMMON STOCK BE ISSUED IN THE MERGER?

No. A Mountain West stockholder will receive cash in lieu of any fractional shares of First Interstate Class A Common Stock such holder would otherwise have been entitled to receive in the merger.

WHAT HAPPENS TO MOUNTAIN WEST STOCK OPTIONS IN THE MERGER?

Under the merger agreement, Mountain West has agreed that every option to purchase Mountain West common stock outstanding and not expired immediately prior to the completion of the merger, whether or not then vested and whether or not then exercisable, shall be cancelled in exchange for a payment in cash equal to (x) the per share merger consideration value, minus (y) the applicable exercise price per share, multiplied by (z) the number of shares available under such option, assuming 100% vesting.

WHAT ARE THE MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER TO MOUNTAIN WEST STOCKHOLDERS?

The merger is intended to qualify, and the obligation of First Interstate to complete the merger is conditioned upon the receipt of a legal opinion from its counsel to the effect that the merger will qualify, as a reorganization within the meaning of Section 368 of the Internal Revenue Code of 1986, as amended, which is referred to herein as the Internal Revenue Code.  In addition, in connection with the filing of the registration statement of which this document is a part, Holland & Hart LLP has delivered an opinion to First Interstate to the same effect.

Assuming the merger qualifies as such a reorganization, a stockholder of Mountain West generally will not recognize any gain or loss upon receipt of First Interstate Class A Common Stock in exchange for Mountain West common stock in the merger but may recognize gain with respect to the cash consideration and cash received in lieu of a fractional share of First Interstate Class A Common Stock.

The consequences of the merger to any particular Mountain West stockholder will depend on that stockholder’s particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the merger and your election statement.

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WHEN WILL THE MERGER BE COMPLETED?

First Interstate and Mountain West are working to complete the merger as soon as practicable. If the stockholders of Mountain West approve the merger agreement and the merger, the parties currently expect that the merger will be completed in mid- 2014, but no later than December 31, 2014.  Neither First Interstate nor Mountain West can predict, however, the actual date on which the merger will be completed because it is subject to factors beyond each company’s control, including whether or when the required regulatory approvals will be received.

WHEN AND WHERE WILL THE MOUNTAIN WEST SPECIAL MEETING OF STOCKHOLDERS TAKE PLACE?

The special meeting of stockholders of Mountain West will be held at 10:00 a.m., Mountain Time, on                       , 2014, at the Holiday Inn — Downtown, 22 N. Last Chance Gulch St., Helena, MT 59601.

WHAT AM I BEING ASKED TO VOTE ON AND WHY IS THIS APPROVAL NECESSARY?

Mountain West stockholders are being asked to vote on the following proposals:

1. to approve the merger agreement, a copy of which is attached as Appendix A to this document, and the merger to be effected pursuant thereto, which are referred to herein collectively as the Mountain West Merger proposal; and

2. to approve one or more adjournments of the Mountain West special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the Mountain West Merger proposal, which is referred to as the Mountain West Adjournment proposal.

Approval by Mountain West stockholders of the Mountain West Merger proposal is required for completion of the merger. Mountain West will transact no other business at the Mountain West special meeting, except for business properly brought before the Mountain West special meeting or any adjournment or postponement thereof.

WHO IS ENTITLED TO VOTE?

Holders of record of Mountain West common stock at the close of business on                      , 2014, which is the date that the Mountain West board of directors has fixed as the record date for the Mountain West special meeting, are entitled to vote at the Mountain West special meeting.

WHAT CONSTITUTES A QUORUM?

The presence at the Mountain West special meeting, in person or by proxy, of holders of a majority of the issued and outstanding shares of Mountain West common stock entitled to vote at the Mountain West special meeting will constitute a quorum for the transaction of business. Abstentions, if any, will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum. So-called broker non-votes, which are described below and not anticipated at this meeting, will have no impact on the determination of a quorum.

WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE MOUNTAIN WEST SPECIAL MEETING?

Mountain West Merger proposal: The affirmative vote of two-thirds of the outstanding shares of Mountain West common stock entitled to vote is required to approve the Mountain West Merger proposal. As described in this proxy statement, all current directors and certain senior executive officers (and their affiliates) of Mountain West have signed agreements to vote in favor of and support the merger. Such persons own approximately     % of outstanding Mountain West common stock as of the record date.

Mountain West Adjournment proposal: Assuming a quorum is present, the affirmative vote of a majority of the shares of Mountain West common stock represented (in person or by proxy) at the Mountain West special meeting and entitled to vote on the proposal is required to approve the Mountain West adjournment proposal.

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WHAT DOES THE MOUNTAIN WEST BOARD OF DIRECTORS RECOMMEND?

The Mountain West board of directors unanimously recommends that Mountain West stockholders vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

WHAT DO I NEED TO DO NOW?

After carefully reading and considering the information contained in this proxy statement/prospectus, if you were the record holder of shares of Mountain West common stock as of                     , 2014, you may vote in person by attending the special meeting or, to ensure that your shares of Mountain West common stock are represented at the special meeting, you may vote your shares over the Internet or by signing and returning the enclosed proxy card in the postage-paid envelope provided. Please follow the instructions set forth on the proxy card or on the voting instruction form provided by the record holder if your shares are held in the name of your broker, bank or other nominee.

HOW DO I VOTE?

If you are a stockholder of record of Mountain West as of                         , 2014, which is referred to as the Mountain West record date, you may submit your proxy before Mountain West’s special meeting in one of the following ways:

·                  visit the website shown on your proxy card to vote via the Internet; or

·                  complete, sign, date and return the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at Mountain West’s special meeting.

If your shares are held in “street name,” through a broker, bank or other nominee, that institution will send you separate instructions describing the procedure for voting your shares. “Street name” stockholders who wish to vote at the meeting will need to obtain a proxy form from their broker, bank or other nominee.

HOW MANY VOTES DO I HAVE AND WHAT PERCENTAGE OF SHARES DO DIRECTORS AND OFFICERS OWN?

You are entitled to one vote for each share of Mountain West common stock that you owned as of the record date. As of the close of business on the record date, there were approximately                shares of Mountain West common stock issued and outstanding and approximately              options to purchase Mountain West common stock outstanding. As of that date, approximately     % of the outstanding shares of Mountain West common stock were beneficially owned by the current directors and certain senior executive officers (and their affiliates) of Mountain West.

IF MY SHARES ARE HELD IN “STREET NAME” BY A BROKER, BANK OR OTHER NOMINEE, WILL MY BROKER, BANK OR OTHER NOMINEE VOTE MY SHARES FOR ME?

If your shares are held in “street name” in a stock brokerage account or by a bank or other nominee, you must provide the record holder of your shares with instructions on how to vote your shares. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote shares held in street name by returning a proxy card directly to Mountain West or by voting in person at Mountain West’s special meeting unless you provide a “legal proxy,” which you must obtain from your broker, bank or other nominee.

Under the rules of the NASDAQ Stock Market, brokers who hold shares in street name for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that the NASDAQ determines to be “non-routine” without specific instructions from the beneficial owner. It is expected that all proposals to be voted on at the Mountain West special meeting are such “non-routine” matters. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of shares to vote on a particular proposal for which the broker does not have discretionary voting power.

Assuming a quorum is present, if you are a Mountain West stockholder and you do not instruct your broker, bank or other nominee on how to vote your shares,

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·                  your broker, bank or other nominee may not vote your shares on the Mountain West Merger proposal, which broker non-votes will have the same effect as a vote “AGAINST” such proposal; and

·                  your broker, bank or other nominee may not vote your shares on the Mountain West Adjournment proposal, which broker non-votes will have no effect on the vote count for such proposal.

WHAT IF I DO NOT VOTE OR ABSTAIN?

Assuming a quorum is present, an abstention occurs when a stockholder attends the special meeting in person and does not vote or returns a proxy with an “abstain” vote.

Assuming a quorum is present, if you are a Mountain West stockholder and you fail to vote or fail to instruct your broker, bank or other nominee how to vote on the Mountain West Merger proposal, it will have the same effect as a vote cast “AGAINST” the Mountain West Merger proposal. If you respond with an “abstain” vote on the Mountain West Merger proposal, your proxy will have the same effect as a vote cast “AGAINST” the Mountain West Merger proposal.

WHAT WILL HAPPEN IF I RETURN MY PROXY OR VOTING INSTRUCTION CARD WITHOUT INDICATING HOW TO VOTE?

If you sign and return your proxy or voting instruction card without indicating how to vote on any particular proposal, the Mountain West common stock represented by your proxy will be voted as recommended by the Mountain West board of directors with respect to that proposal. Unless a Mountain West stockholder, as applicable, checks the box on its proxy card to withhold discretionary authority, the proxyholders may use their discretion to vote on other matters that may properly come before the meeting, if any the Mountain West special meeting.

MAY I CHANGE MY VOTE AFTER I HAVE DELIVERED MY PROXY OR VOTING INSTRUCTION CARD?

Yes. You may change your vote at any time before your proxy is voted at the Mountain West special meeting. You may do this in one of four ways:

·                  by sending a notice of revocation to the corporate secretary of Mountain West;

·                  by logging onto the Internet website specified on your proxy card in the same manner you would to submit your proxy electronically if you are eligible to do so and following the instructions on the proxy card;

·                  by sending a completed proxy card bearing a later date than your original proxy card; or

·                  by attending the Mountain West special meeting and voting in person.

If you choose any of the first three methods, you must take the described action such that the notice, internet vote or proxy card, as applicable, is received no later than the beginning of the special meeting.

If your shares are held in an account at a broker, bank or other nominee, you should contact your broker, bank or other nominee to change your vote.

DO I NEED IDENTIFICATION TO ATTEND THE MOUNTAIN WEST MEETING IN PERSON?

Yes. Please bring proper identification, together with proof that you are a record owner of Mountain West common stock. If your shares are held in street name, please bring acceptable proof of ownership, such as a letter from your broker or an account statement showing that you beneficially owned shares of Mountain West common stock, as applicable, on the record date.

ARE MOUNTAIN WEST STOCKHOLDERS ENTITLED TO DISSENTERS’ RIGHTS?

Under Montana law, Mountain West stockholders are entitled to dissenters’ rights in connection with the merger.

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WHAT HAPPENS IF THE MERGER IS NOT COMPLETED?

If the merger is not completed, Mountain West stockholders will not receive any consideration for their shares of Mountain West common stock in connection with the merger. Instead, Mountain West will remain an independent private company and its common stock will continue to be unlisted. Under specified circumstances described elsewhere herein, Mountain West may be required to pay First Interstate a fee in connection with the termination of the merger agreement.

SHOULD MOUNTAIN WEST STOCKHOLDERS SEND IN THEIR STOCK CERTIFICATES NOW?

No. Mountain West stockholders SHOULD NOT send in any stock certificates now. If the merger is approved, a letter of transmittal with instructions for its completion will be provided to Mountain West stockholders under separate cover and the stock certificates should be sent at that time.

WHOM SHOULD I CONTACT IF I HAVE ANY QUESTIONS ABOUT THE PROXY MATERIALS OR VOTING?

If you are a Mountain West stockholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact Mountain West Investor Relations at 406-449-2265.

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SUMMARY

This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents referred to before you decide how to vote with respect to the merger-related proposals. In addition, important business and financial information about First Interstate is incorporated by reference into this document. For a description of this information, please see the section entitled “INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE” beginning on page 79. You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled “WHERE YOU CAN FIND MORE INFORMATION” in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

Unless the context otherwise requires, throughout this document, “First Interstate” refers to First Interstate BancSystem, Inc., “Mountain West” refers to Mountain West Financial Corp. and “the combined company” refers collectively to First Interstate and Mountain West. Also, the proposed merger of Mountain West with and into First Interstate is referred to as, the “merger,” the proposed merger of Mountain West Bank with and into First Interstate Bank as the “bank merger,”  and the Agreement and Plan of Merger, dated as of February 10, 2014, by and between First Interstate and Mountain West as the “merger agreement.”

Information about the Companies (page 26)

First Interstate BancSystem, Inc.
401 North 31st Street
Billings, Montana
Phone: (406) 255-5390

First Interstate is a financial and bank holding company headquartered in Billings, Montana and registered under the Bank Holding Company Act of 1956, as amended, which is referred to herein as the BHC Act. As of September 30, 2013, First Interstate had consolidated assets of $7.5 billion, deposits of $6.1 billion, loans of $4.3 billion and total stockholders’ equity of $793 million. As of the date of this document, First Interstate operates 74 banking offices, including detached drive-up facilities, in 41 communities located in Montana, Wyoming and western South Dakota. Through First Interstate Bank, First Interstate delivers a comprehensive range of banking products and services to individuals, businesses, municipalities and other entities throughout First Interstate’s market areas. First Interstate’s customers participate in a wide variety of industries, including energy, tourism, agriculture, healthcare, professional services, education, governmental services, construction, mining, retail and wholesale trade.

Mountain West Financial Corp.

1225 Cedar Street

Helena, Montana
Phone: (406) 449-2265

Mountain West is a bank holding company headquartered in Helena, Montana and registered under the BHC Act.  As of September 30, 2013, Mountain West had consolidated assets of $647 million, deposits of $542.5 million, loans of $402 million and total stockholders’ equity of $60.3 million.  As of the date of this document, Mountain West operates 12 branches in five primary markets in Montana — Helena, Great Falls, Missoula, the Flathead Valley (Kalispell and Whitefish) and Bozeman. Through Mountain West Bank, Mountain West offers a full range of commercial and consumer banking products and services to individuals, businesses and organizations in Montana.

The Merger and the Merger Agreement (pages 27 and 48)

The terms and conditions of the merger are contained in the merger agreement, which is attached to this document as Appendix A. You are encouraged you to read the merger agreement carefully, as it is the legal document that governs the merger.  Under the terms of the merger agreement, Mountain West will merge with and into First Interstate with First Interstate as the surviving corporation.

Merger Consideration (page 27)

Under the merger agreement, at the effective time of the merger, each issued and outstanding share of Mountain West

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common stock will be cancelled and converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash or, if a Mountain West stockholder properly elects and subject to the limitations summarized below, an amount in all cash or all stock roughly equal in value to the above described combination of stock and cash merger consideration.  Based on the closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market on                               , 2014, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on such date, the aggregate value of the consideration that would be expected to be delivered to Mountain West stockholders in the merger would be approximately $      million, of which $      million would be paid in cash and $      million would be represented by                         shares of First Interstate Class A Common Stock.  The actual value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the First Interstate Class A Common Stock. You may obtain current stock price quotations for First Interstate Class A Common Stock on the NASDAQ Global Select Market under the symbol “FIBK.”  Mountain West common stock is not listed on a national securities exchange, and therefore current Mountain West stock price quotations are not available.

As an accommodation to Mountain West stockholders, First Interstate has agreed to allow them to elect to receive their individual merger consideration in either all cash or all First Interstate Class A Common Stock, subject to the limitations discussed below and in lieu of the stock and cash combination merger consideration.  Electing Mountain West stockholders may not ultimately receive in the merger only cash or only stock, however, because the merger agreement provides that First Interstate is only required to issue that number of shares that equals the target share number.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock at the target share number, then the Mountain West shares shall be converted in accordance with the elections.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock in excess of the target share number, then the number of shares of First Interstate Class A Common Stock to be delivered to the Mountain West stockholders making the all stock elections will be reduced proportionately and cash in an equivalent value will be paid to those stockholders.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of fewer shares of First Interstate Class A Common Stock than the target share number, then the amount of cash to be delivered to the Mountain West stockholders making the all cash elections will be reduced proportionately and shares of First Interstate Class A Common Stock having an equivalent value will be issued to those stockholders.

Recommendation of the Mountain West Board of Directors and Reasons for the Merger (page 33)

The Mountain West board of directors unanimously recommends that Mountain West stockholders vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

For a more complete description of Mountain West’s reasons for the merger and the recommendation of the Mountain West board of directors, please see the section entitled “THE MERGER—Recommendation of the Mountain West Board of Directors and Reasons for the Merger” beginning on page 33.

Mountain West Financial Advisor

On February 10, 2014, D.A. Davidson & Co., which is referred to herein as Davidson, Mountain West’s financial advisor in connection with the merger, delivered an oral fairness opinion to Mountain West’s board of directors, which was subsequently confirmed in a written opinion dated the same date that, as of such date and subject to and based on the qualifications and assumptions set forth in its written opinion, the per share merger consideration in the proposed merger was fair, from a financial point of view, to the stockholders of Mountain West.  In addition, on the date of this prospectus/proxy statement, Davidson provided an updated fairness opinion to Mountain West’s board of directors that as of such date and subject to and based on the qualifications and assumptions set forth in its written opinion, the per share merger consideration in the proposed merger was fair, from a financial point of view, to the stockholders of Mountain West.

The full text of Davidson’s opinions, dated February 10, 2014, and                        , 2014, are attached as Appendices B and          , respectively, to this document. You should read the opinions in their entirety for a description of the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Davidson in rendering its opinion.

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Davidson’s opinions are addressed to Mountain West’s board of directors and the opinions are not a recommendation as to how any stockholder of Mountain West should vote with respect to the merger or any other matter or as to any action that a stockholder should take with respect to the merger.

The opinions address only the fairness, from a financial point of view, of the per share merger consideration in the proposed merger to the stockholders of Mountain West, and do not address the underlying business decision of Mountain West to engage in the merger, or the relative merits of the merger as compared to any strategic alternatives that may be available to Mountain West.  Davidson has received a fee for its services.

For further information, please see the section entitled “THE MERGER—Opinion of Mountain West’s Financial Advisor” beginning on page 35.

Mountain West Special Meeting of Stockholders (page 22)

The Mountain West special meeting will be held at 10:00 a.m., Mountain Time, on                     , 2014, at the Holiday Inn — Downtown, 22 N. Last Chance Gulch St., Helena, MT 59601. At the Mountain West special meeting, Mountain West stockholders will be asked to approve the Mountain West Merger proposal and the Mountain West Adjournment proposal.

Mountain West’s board of directors has fixed the close of business on                      , 2014 as the record date for determining the holders of Mountain West common stock entitled to receive notice of and to vote at the Mountain West special meeting. Only holders of record of Mountain West common stock at the close of business on the Mountain West record date will be entitled to notice of and to vote at the Mountain West special meeting and any adjournment or postponement thereof, unless the adjournment is for more than 30 days, or if after the adjournment a new record date is fixed for the adjourned meeting, a notice of the adjourned meeting shall be given to each stockholder of record entitled to vote at such meeting.

As of the record date, there were                      shares of Mountain West common stock outstanding held by approximately         holders of record and approximately                options to purchase Mountain West common stock outstanding.  Each share of Mountain West common stock entitles the holder to one vote on each proposal to be considered at the Mountain West special meeting. All current directors and certain senior executive officers (and their affiliates) of Mountain West have signed agreements to vote in favor of and support the merger. Such persons own approximately       % of outstanding Mountain West common stock as of the record date.  As of the record date, First Interstate and its executive officers and directors beneficially held no shares of Mountain West’s common stock.

Financial Interests of Directors and Officers of Mountain West in the Merger (page 43)

Certain of Mountain West’s executive officers and directors have financial interests in the merger that are different from, or in addition to, the interests of Mountain West’s stockholders. These interests include:

·                  Mr. Bradshaw is partially vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at age 62 (or if later, his retirement).  If Mr. Bradshaw’s employment is terminated within 24 months after the merger, Mr. Bradshaw will be fully vested in the payments, such that he would receive monthly payments over 20 years at an annual rate of $36,000 beginning immediately after his termination of employment.

·                  Mr. Brown is partially vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at age 65 (or if later, his retirement).  If Mr. Brown’s employment is terminated within 24 months after the merger, Mr. Brown will instead receive a lump sum payment of $77,600, $159,856, or $246,977 (depending on the date of termination) immediately after his termination of employment.

·                  Pursuant to the terms of his 2002 Employment Agreement, Mr. Morgan will be entitled to a lump sum payment of $532,223 on account of the merger, payable upon the closing of the merger.

·                  Mr. Morgan is fully vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at his retirement; however, if Mr. Morgan’s employment is terminated within 24 months after the merger, Mr. Morgan would instead receive a lump sum amount equal to $777,515 at the time of his termination of employment.

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·                  The directors and officers of Mountain West and Mountain West Bank will receive continued indemnification and director and officer liability insurance coverage for a period of six years after completion of the merger.

Treatment of Mountain West Stock Options (page 31)

Under the merger agreement, Mountain West has agreed that every option to purchase Mountain West common stock outstanding and not expired immediately prior to the completion of the merger, whether or not then vested and whether or not then exercisable, shall be cancelled in exchange for a payment in cash equal to (x) the per share merger consideration value, minus (y) the applicable exercise price per share, multiplied by (z) the number of shares available under such option, assuming 100% vesting.

Regulatory Approvals Required for the Merger (page 45)

Completion of the merger and the bank merger are subject to various regulatory approvals, including the Board of Governors of the Federal Reserve System, which is referred to herein as Federal Reserve, and the Montana Division of Banking and Financial Institutions. Notifications and/or applications requesting approval for the merger or for the bank merger may also be submitted to other federal and state regulatory authorities. First Interstate and Mountain West have filed, or are in the process of filing, notices and applications to obtain the necessary regulatory approvals. Although First Interstate and Mountain West currently believe they should be able to obtain all required regulatory approvals in a timely manner, they cannot be certain when or if they will obtain them or, if obtained, whether they will contain terms, conditions or restrictions not currently contemplated that will be detrimental to or have a material adverse effect on First Interstate after the completion of the merger. The regulatory approvals to which completion of the merger and bank merger are subject are described in more detail under the section entitled “THE MERGER—Regulatory Approvals Required for the Mergers” beginning on page 46.  Regulatory approvals are also required for Mountain West to pay those dividends to its stockholders that it is permitted to pay pursuant to the merger agreement.

Conditions to the Merger (page 57)

Among other things, the obligations of First Interstate and Mountain West to complete the merger are each subject to the satisfaction or waiver of the following conditions:

·                  approval of the Mountain West Merger proposal by the Mountain West stockholders;

·                  the effectiveness of the registration statement on Form S-4, of which this document is a part, and the absence of a stop order or proceeding initiated or threatened by the SEC for that purpose;

·                  the accuracy of the representations and warranties of each party as of the closing date of the merger;

·                  performance in all material respects by each party of the obligations required to be performed by it at or prior to the closing date of the merger;

·                  receipt of all consents and approvals required to consummate the merger; and

·                  receipt by First Interstate of an opinion of its tax counsel as to certain tax matters.

No Solicitation and Change in Recommendation (page 52)

Under the terms of the merger agreement, Mountain West has agreed not to solicit, initiate, encourage or facilitate inquiries or proposals with respect to, participate in any discussions or negotiations concerning, or enter into any agreement regarding any acquisition proposal. Notwithstanding these restrictions, the merger agreement provides that, under specified circumstances and with advance notice to First Interstate, in response to an unsolicited bona fide acquisition proposal which, in the good faith judgment of the Mountain West board of directors, is or is reasonably likely to result in a proposal which is superior to the merger with First Interstate, and the Mountain West board of directors determines in good faith (and after consultation with Mountain West’s outside counsel) that failure to take such actions would reasonably be expected to be a violation of its fiduciary duties under applicable law, Mountain West may furnish information regarding Mountain West and participate in discussions and negotiations with such third party.

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Termination of the Merger Agreement (page 53)

First Interstate and Mountain West may mutually agree at any time to terminate the merger agreement without completing the merger, even if the Mountain West stockholders have voted to approve the merger agreement.  The merger agreement may also be terminated and the merger abandoned at any time prior to the effective time of the merger, as follows:

·                  by either First Interstate or Mountain West, if the merger is not completed on or before December 31, 2014, unless the failure of the closing to occur by such date is due to a breach of a representation, warranty, covenant or agreement by the party seeking to terminate the merger agreement;

·                  by Mountain West, if the average closing price of First Interstate’s Class A Common Stock on the NASDAQ Global Market is less than $20.00 per share during the twenty (20) business days preceding the fifth business day prior to the closing of the merger, which is referred to herein as the average closing price, unless First Interstate, within five (5) business days of receiving a notice of termination by Mountain West, adjusts the merger consideration by an amount equal to the difference between $20.00 and the average closing price, multiplied by the number of shares of First Interstate Class A Common Stock to be issued in the merger;

·                  by either First Interstate or Mountain West, if a required regulatory approval is denied and First Interstate elects not to appeal such denial within fifteen (15) business days of such denial;

·                  by either First Interstate or Mountain West, if the stockholders of Mountain West have not approved the merger at the Mountain West special meeting or any adjournment or postponement thereof;

·                  by either First Interstate or Mountain West, as applicable, if the closing conditions of the other party are not satisfied or waived or become impossible to satisfy (other than as a result of the terminating party to comply with its obligations under the merger agreement) as of December 31, 2014;

·                  by First Interstate, if environmental site assessments indicate a material adverse environmental condition that First Interstate reasonably determines would have a material adverse effect on Mountain West on a consolidated basis and Mountain West or its subsidiary is unable to remedy the condition within sixty (60) days after First Interstate delivers notice thereof;

·                  subject to certain limitations, by Mountain West, if Mountain West’s board of directors determines an acquisition proposal is a superior proposal and delivers a break-up fee equal to $3 million by wire transfer of immediately available funds to First Interstate; or

·                  by either First Interstate or Mountain West, as applicable, if the other party breaches any of its representations, warranties, agreements or covenants set forth in the merger agreement, which breach is not, or cannot be, cured within thirty (30) days following notice to the breaching party.

Mountain West may be required to pay First Interstate a termination fee equal to $1 million or a break-up fee equal to $3 million in certain circumstances. For more information, please see the section entitled “THE MERGER AGREEMENT — Termination of Merger Agreement —Termination Fee and Break-up Fee” beginning on page 54.

Material United States Federal Income Tax Consequences of the Merger (page 62)

The merger is intended to qualify as a reorganization within the meaning of Section 368 of the Internal Revenue Code. Assuming the merger qualifies as such a reorganization, a stockholder of Mountain West generally will not recognize any gain or loss upon receipt of First Interstate Class A Common Stock in exchange for Mountain West common stock in the merger, but may recognize gain with respect to the cash consideration and cash received in lieu of a fractional share of First Interstate Class A Common Stock. It is a condition to the completion of the merger that First Interstate receives a written opinion from its counsel to the effect that the merger will qualify as a reorganization within the meaning of Section 368 of the Internal Revenue Code.

Tax matters are very complicated and the tax consequences of the merger to each Mountain West stockholder may depend on such stockholder’s particular facts and circumstances. Mountain West stockholders are urged to consult their tax

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advisors to understand fully the tax consequences to them of the merger. For more information, please see the section entitled “MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER” beginning on page 62.

Comparison of Stockholders’ Rights (page 73)

The rights of Mountain West stockholders who continue as First Interstate stockholders after the merger will be governed by the Amended and Restated Articles of Incorporation and Second Amended and Restated Bylaws of First Interstate rather than by the articles of incorporation and bylaws of Mountain West. For more information, please see the section entitled “COMPARISON OF STOCKHOLDERS’ RIGHTS” beginning on page 73.

Dissenters’ Rights of Mountain West Stockholders (page 66)

As a stockholder of Mountain West, under Montana law you have the right to dissent from the merger and have the fair value of your shares of Mountain West common stock paid to you in cash. The fair value may be more or less than the value of the shares of First Interstate Class A Common Stock and cash being paid in the merger.

Persons having beneficial interests in Mountain West common stock held of record in the name of another person, such as a broker or bank, must act promptly to cause the record holder to take the actions required under Montana law to exercise your dissenters’ rights.

In order to dissent, you must carefully follow the requirements of the Montana Code Annotated, including giving the required written notice prior to the special meeting at which the vote on the merger agreement is taken. These steps are summarized under the caption “DISSENTERS’ RIGHTS OF MOUNTAIN WEST STOCKHOLDERS” on page 66.

If you intend to exercise dissenters’ rights, you should read the statutes carefully and consult with your own legal counsel. You should also remember that if you return a signed proxy card but fail to provide instructions as to how your shares of Mountain West common stock are to be voted, you will be considered to have voted in favor of the merger agreement and you will not be able to assert dissenters’ rights. Also, if you exercise dissenters’ rights, you may have taxable income as a result, so you are urged to consult with your own tax advisor if you intend to dissent. See the section entitled “MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER” on page 62. If the merger agreement is approved by the stockholders of Mountain West, holders of Mountain West common stock who make a written objection to the merger prior to the Mountain West special meeting, vote against the approval of the merger agreement, properly make a written demand for payment following notice of the merger and timely surrender their Mountain West stock certificates will be entitled to receive the fair value of their shares in cash under the Montana Code Annotated.

The text of the provisions of the Montana Code Annotated pertaining to dissenters’ rights is attached to this proxy statement/prospectus as Appendix C.

Risk Factors (page 19)

Before voting at the Mountain West special meeting, you should carefully consider all of the information contained in or incorporated by reference into this proxy statement/prospectus, including the risk factors set forth in the section entitled “RISK FACTORS” beginning on page 19 or described in First Interstate’s Annual Report on Form 10-K as of and for the year ended December 31, 2012 and other reports filed with the SEC, including subsequent quarterly reports filed on Form 10-Q, which are incorporated by reference into this proxy statement/prospectus. Please see the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii.

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SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA FOR FIRST INTERSTATE

The following table summarizes consolidated financial results achieved by First Interstate for the periods and at the dates indicated and should be read in conjunction with First Interstate’s consolidated financial statements and the notes to the consolidated financial statements contained in reports that First Interstate has previously filed with the SEC. Historical financial information for First Interstate can be found in its Quarterly Report on Form 10-Q for the quarter ended September 30, 2013 and its Annual Report on Form 10-K for the year ended December 31, 2012. Please see the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii for instructions on how to obtain the information that has been incorporated by reference. Financial amounts as of and for the nine months ended September 30, 2013 are unaudited (and are not necessarily indicative of the results of operations for the full year or any other interim period), and management of First Interstate believes that such amounts reflect all adjustments (consisting only of normally recurring adjustments) necessary for a fair presentation of its results of operations and financial position as of the dates and for the periods indicated. You should not assume the results of operations for the past periods presented indicate results for any future period.

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(1) For purposes of computing book value per share, book value equals common stockholders’ equity.

(2) Net interest margin ratio is presented on a fully taxable equivalent basis.

(3) Efficiency ratio represents non-interest expenses, excluding loan loss provision, divided by the aggregate of net interest income and non-interest income.

(4) Common stock dividend payout ratio represents dividends per share divided by basic earnings per share.

(5) Non-performing loans include nonaccrual loans (including nonaccrual troubled debt restructurings) and loans past due 90 days or more and still accruing interest.

(6) Non-performing assets include nonaccrual loans (including nonaccrual troubled debt restructurings), loans past due 90 days or more, and OREO.

(7) For purposes of computing tier 1 common capital to total risk weighted assets, tier 1 common capital is calculated as tier 1 capital less preferred stock and trust preferred securities.

(8) In January 2013, First Interstate changed its definition of non-performing loans and the ratios included in the table above to recast to conform to the revised definition.  As a result, its historical financial information for prior periods in documents incorporated by reference herein will not reflect this change in definition. For such prior periods and per such prior definition, the ratios would have been as follows:

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COMPARATIVE PER SHARE MARKET PRICE AND DIVIDEND INFORMATION

The tables below set forth, for the calendar quarters indicated, the high and low sales prices per share for, and the dividend paid per share with respect to, First Interstate Class A Common Stock, which is listed for trading on the NASDAQ Global Select Market under the symbol “FIBK,” and Mountain West common stock, for which no established public trading market exists. Though sporadic trading occurs in an over-the-counter market for Mountain West common stock, no registered broker/dealer makes a market in Mountain West common stock, and its over-the-counter market quotations reflect inter-dealer prices, without retail mark-up, mark-down or commission and may not necessarily represent actual transactions. The following table setting forth the ranges of trading prices for Mountain West’s common stock for the periods presented is provided to the extent known or believed to be known to the management of Mountain West.

* This dividend was declared by the Mountain West board of directors on December 30, 2013 and is to be paid on April 1, 2014 to stockholders of record as of February 28, 2014.

The following table sets forth the closing sale prices per share of First Interstate Class A Common Stock and Mountain West common stock on February 10, 2014, the last trading day before the public announcement of the signing of the merger agreement, and on                    , 2014, the latest practicable date before the date of this document.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

Some of the statements in this document and the documents incorporated by reference herein contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or Securities Act, and Rule 175 promulgated thereunder, and Section 21E of the Securities Exchange Act of 1934, as amended, or Exchange Act, and Rule 3b-6 promulgated thereunder, that involve inherent risks and uncertainties.  These statements include statements about First Interstate’s and Mountain West’s plans, strategies and prospects and involve known and unknown risks that are difficult to predict. Therefore, actual results, performance or achievements may differ materially from those expressed in or implied by these forward-looking statements. In some cases, you can identify forward-looking statements by the use of words such as “may,” “could,” “expect,” “intend,” “plan,” “seek,” “anticipate,” “believe,” “estimate,” “predict,” “potential,” “continue,” “likely,” “will,” “would” and variations of these terms and similar expressions, or the negative of these terms or similar expressions.  You should carefully read forward-looking statements, including statements that contain these words, because they discuss the future expectations or state other “forward-looking” information about First Interstate, Mountain West and the combined company. Forward-looking statements speak only as of the date they are made. First Interstate and Mountain West assume no duty to update such statements.

Factors that may cause actual results to differ materially from current expectations are described in First Interstate’s reports filed with the SEC and the section of this document entitled “RISK FACTORS” (beginning on page 19), and include, but are not limited to:

·                  the ability to obtain regulatory approvals and meet other closing conditions to the merger, including approval by Mountain West stockholders, on the expected terms and schedule;

·                  a delay in closing the merger;

·                  difficulties and delays in integrating the First Interstate and Mountain West businesses or fully realizing cost savings and other benefits, including as a result of (i) potential negative publicity regarding consolidations or reductions in force, (ii) an inability to retain Mountain West employees, or (iii) attrition of Mountain West Bank customers;

·                  business disruption following the merger;

·                  diversion of management’s attention from ongoing business operations and opportunities;

·                  worsening economic conditions;

·                  adverse economic conditions affecting Montana, Wyoming and western South Dakota;

·                  credit losses;

·                  concentrations of real estate loans;

·                  commercial loan risk;

·                  adequacy of the allowance for loan losses;

·                  impairment of goodwill;

·                  changes in interest rates;

·                  access to low-cost funding sources;

·                  increases in deposit insurance premiums;

·                  inability to grow First Interstate’s business;

·                  the impact, extent and timing of technological changes, capital management activities, and other actions of the

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Federal Reserve, the Department of Business Oversight and the FDIC, and legislative and regulatory actions and reforms;

·                  governmental regulation and changes in regulatory, tax and accounting rule and interpretations;

·                  sweeping changes in regulation of financial institutions due to passage of Dodd-Frank Act;

·                  changes in or noncompliance with governmental regulations;

·                  effects of recent legislative and regulatory efforts to stabilize financial markets;

·                  dependence on management teams;

·                  ability to attract and retain qualified employees;

·                  failure of technology;

·                  reliance on external vendors;

·                  inability to meet liquidity requirements;

·                  lack of acquisition candidates;

·                  failure to manage growth;

·                  competition;

·                  inability to manage risks in turbulent and dynamic market conditions;

·                  ineffective internal operational controls;

·                  environmental remediation and other costs;

·                  litigation pertaining to fiduciary responsibilities;

·                  failure to effectively implement technology-driven products and services;

·                  capital required to support the bank subsidiaries, First Interstate Bank and Mountain West Bank;

·                  soundness of other financial institutions;

·                  impact of proposed Basel III capital standards for U.S. banks;