FIRST INTERSTATE BANCSYSTEM INC

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A INFORMATION

(Rule 14a-101)

Proxy Statement Pursuant to Section 14(a)

of the Securities Exchange Act of 1934

(Amendment No. ____)

Filed by the Registrant ☒

Filed by a Party other than the Registrant☐

Check the appropriate box:

☐Preliminary Proxy Statement

☐Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☒ Definitive Proxy Statement

☐Definitive Additional Materials

☐Soliciting Material Pursuant to §240.14a-12

FIRST INTERSTATE BANCSYSTEM, INC.

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

2026 Proxy Statement

First Interstate BancSystem, Inc.

Notice of Annual Meeting of Shareholders to be held on May 27, 2026

Notice of Annual Meeting of Shareholders

Participate in the Future of First Interstate — Please Cast Your Vote

At the 2026 Annual Meeting of Shareholders (the “annual meeting”) of First Interstate BancSystem,

Inc., shareholders will be asked to vote on the following proposals:

1. To elect three directors;

2. To approve an amendment to our Certificate of Incorporation (the “Charter”) to provide for

plurality voting in contested director elections;

3. To approve, on a non-binding, advisory basis, the compensation of the Company’s Named

Executive Officers; and

4. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting

firm for the year ending December 31, 2026.

YOUR VOTE IS IMPORTANT TO US. PLEASE CAST YOUR VOTE AS SOON AS POSSIBLE. Shareholders of

record as of the close of business on Thursday, April 2, 2026, are entitled to notice of and to vote at

the annual meeting and any adjournments or postponements thereof. Whether or not you plan to

attend the annual meeting, we urge you to vote as soon as possible. A proxy that is signed and dated,

but which does not contain voting instructions, will be voted in the manner as is recommended by our

Board of Directors on each proposal with respect to which a registered holder is entitled to vote.

Registered holders may vote:

▪By Internet — access http://www.voteproxy.com and follow the on-screen instructions;

▪By mail — sign, date, and mail your proxy card in the envelope provided as soon as possible, if

you received a paper copy of the proxy materials; or

▪In person — vote your shares in person by attending the annual meeting.

Questions and Answers about the 2026 Annual Meeting. We encourage you to review the section

captioned “Information About the Shareholder Meeting” beginning on page 97 for answers to common

questions about the annual meeting, proxy materials, voting, and other related topics.

BY ORDER OF THE BOARD OF DIRECTORS

Kirk D. Jensen

General Counsel and Corporate Secretary

Billings, Montana

April 16, 2026

Important Notice Regarding the Availability of Proxy Materials for the 2026 Annual Meeting of

Shareholders

to be Held on May 27, 2026 at 4:00 p.m., Mountain Time.

The proxy statement and annual report to shareholders are available at

www.astproxyportal.com/ast/40019/.

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