8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (date of earliest event reported): August 29, 2025

FIRST INTERSTATE BANCSYSTEM, INC.

(Exact name of registrant as specified in its charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a- 12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

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Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

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Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On August 29, 2025, the Board of Directors (the “Board”) of First Interstate BancSystem, Inc. (the “Company”) appointed Ms. Renu Agrawal to serve as a Class III director on the Board with a term expiring at the Company’s 2027 annual meeting of shareholders. Ms. Agrawal will fill a vacancy on the Board created by the Board’s decision to increase the size of the Board.

Ms. Agrawal spent 13 years at Wells Fargo, leading strategic initiatives at the corporate level as well as running multiple divisions. Prior to that, she was Chief Operating Officer for both ValleyCrest Companies and Quisic Corporation. Renu started her career as a scientist at Polaroid and was at McKinsey for several years. She previously served on the Boards of Luther Burbank Corporation and Woodruff Sawyer.

There was no arrangement or understanding pursuant to which Ms. Agrawal was appointed as a director, and since the beginning of the Company’s last fiscal year, Ms. Agrawal has not engaged in any transaction with the Company that would be reportable as a related person transaction under Item 404(a) of Regulation S-K.

The Board has determined that Ms. Agrawal will be “independent” as defined under applicable NASDAQ Marketplace Rules at the time of her appointment. Ms. Agrawal will be entitled to receive the same compensation for service as a director as is provided to other non-employee directors of the Company pursuant to the Company’s non-employee director compensation program, which includes the issuance of restricted stock units, as described under the heading “Director Compensation” in the Company’s proxy statement for its 2025 annual meeting of shareholders filed with the Securities and Exchange Commission on April 8, 2025. Ms. Agrawal has been appointed to serve on the Board’s Governance and Nominating Committee and Technology, Innovation and Operations Committee.

Following Ms. Agrawal’s appointment, as of August 29, 2025, there are currently 14 members on the Board, five of whom are designated as Class I directors, four of whom are designated as Class II directors, and five of whom are designated as Class III directors.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: September 2, 2025