UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington D.C. 20549

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FORM 8-K/A

Amendment No. 1

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CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of report (date of earliest event reported): February 1, 2022

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FIRST INTERSTATE BANCSYSTEM, INC.

(Exact name of registrant as specified in its charter)

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(406) 255-5390

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

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):

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. ☐

Explanatory Note

This Amendment No. 1 to Current Report on Form 8-K/A is being filed with the Securities and Exchange Commission (the “Commission”) solely to amend and supplement Item 9.01 of the Current Report on Form 8-K filed by First Interstate BancSystem, Inc. (“First Interstate” or the “Company”) on February 1, 2022, regarding the completion of its merger (the “Merger”) with Great Western Bancorp, Inc. (“Great Western”) on February 1, 2022, pursuant to the Agreement and Plan of Merger, dated as of September 15, 2021 (the “Merger Agreement”), by and between First Interstate and Great Western. As noted below, First Interstate has determined that the financial statements of Great Western are not required in the Current Report on Form 8-K, as amended by this Current Report on Form 8-K/A, because “substantially the same” information was previously reported, as contemplated under Rule 12b-2 of the Securities Exchange Act of 1934, as amended, in the Joint Proxy Statement/Prospectus forming a part of First Interstate’s Registration Statement on Form S-4, as amended (File No. 333- 260771), initially filed on November 24, 2021 and declared effective by the Commission on December 16, 2021 (the “Registration Statement”).

(a) Financial Statements of Business Acquired.

Pursuant to General Instruction B.3 of Form 8-K, the financial statements of the acquired company, Great Western, are not required in the Current Report on Form 8-K, as amended by this Current Report on Form 8-K/A, because “substantially the same” financial statements were previously filed in the Registration Statement.

(b) Pro forma financial information.

The unaudited pro forma combined condensed statement of income for the year ended December 31, 2021, giving effect to the Merger as if it had been consummated on January 1, 2021, the first day of the Company’s 2021 fiscal year, the unaudited pro forma combined condensed balance sheet as of December 31, 2021, giving effect to the Merger as if it had been consummated on December 31, 2021, and the related notes are attached as Exhibit 99.2 hereto and are incorporated herein by reference.

(d) Exhibits

*Previously filed as an exhibit to the Original Filing

SIGNATURE

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: April 19, 2021