8-K/A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington D.C. 20549

FORM 8-K/A

(Amendment No. 1)

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 13, 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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Explanatory Note

On August 15, 2025, First Interstate BancSystem, Inc. (the “Company”) filed a Current Report on Form 8-K (the “Original Form 8-K”) with the Securities and Exchange Commission (“SEC”) announcing that its board of directors (the “Board”) appointed Mr. Michael L. Scudder to serve as a Class I director on the Board, with a term beginning on August 13, 2025, and expiring at the Company’s 2028 annual meeting of stockholders, to fill a vacancy on the Board created by the Board’s decision to increase the size of the Board. At the time the Original Form 8-K was filed, the Board had not yet made a determination regarding Board committee assignments for Mr. Scudder. This Amendment No. 1 to the Original Form 8-K is being filed solely to report the Board committees to which Mr. Scudder has been assigned, as required by the SEC. Except as set forth herein, the Original Form 8-K remains unchanged in all respects.

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

Effective August 28, 2025, the Board appointed Mr. Scudder to serve as a member of the Board’s Audit Committee and Risk Committee.

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: August 29, 2025