Chief Operations Officer — Jeff Lee: The filing announces the appointment of an external candidate to the senior executive role of Chief Operations Officer, which is a new hire rather than a departure.

Results of Operations and Financial Condition. On July 23, 2026 , First Interstate BancSystem, Inc. (the “Company”) issued a press release regarding its financial results for the quarter ended June 30, 2026. A copy of the press release is furnished herewith as Exhibit 99.1 and is incorporated by reference herein. Neither the information included or incorporated by reference under this Item 2.02, nor the press release furnished herewith, shall be deemed to be “filed” for purposes of Section 18…

Other Events. On July 23, 2026 , the Company also announced that the Board of Directors of the Company declared, on July 22, 2026, a dividend of $0.47 per share, that is payable August 14, 2026 to shareholders of record of the Company as of August 4, 2026.

Class II directors on the Board — Mr. Matthew Ritter and Mr. Kevin Turner: The Board of Directors appointed two new Class II directors to the board, expanding its size and adding experienced leadership.

Executive Vice President and General Counsel/Corporate Secretary — Mr. Kirk D. Jensen: The employment of Mr. Kirk D. Jensen was terminated involuntarily and without cause.

Director — Ms. Patricia L. Moss, Messrs. David L. Jahnke and Stephen M. Lacy: Directors automatically retired due to reaching the retirement age of 72.

Executive Advisor to the CEO — Kristina Robbins: Ms. Kristina Robbins' role was transitioned from COO to Executive Advisor to assist with the transition and provide strategic continuity.

Other Events. On April 29, 2026, the Company also announced that the Board of Directors of the Company declared, on April 28, 2026, a dividend of $0.47 per share, that is payable May 21, 2026 to shareholders of record of the Company as of May 11, 2026.

Results of Operations and Financial Condition. On April 29, 2026, First Interstate BancSystem, Inc. (the “Company”) issued a press release regarding its financial results for the quarter ended March 31, 2026. A copy of the press release is furnished herewith as Exhibit 99.1 and is incorporated by reference herein. Neither the information included or incorporated by reference under this Item 2.02, nor the press release furnished herewith, shall be deemed to be “filed” for purposes of Section 1…

Other Events. On January 28, 2026, the Company also announced that the Board of Directors of the Company declared, on January 27, 2026, a dividend of $0.47 per share, that is payable February 20, 2026 to shareholders of record of the Company as of February 10, 2026.

Results of Operations and Financial Condition. On January 28, 2026, First Interstate BancSystem, Inc. (the “Company”) issued a press release regarding its financial results for the quarter ended December 31, 2025. A copy of the press release is furnished herewith as Exhibit 99.1 and is incorporated by reference herein. Neither the information included or incorporated by reference under this Item 2.02, nor the press release furnished herewith, shall be deemed to be “filed” for purposes of Sect…

Co-Chief Banking Officer — Lorrie F. Asker: The Co-Chief Banking Officer is transitioning to an advisor role to facilitate an orderly succession to the other Co-CBO, who is being appointed as the sole Chief Banking Officer.

Results of Operations and Financial Condition. On October 29, 2025, First Interstate BancSystem, Inc. (the “Company”) issued a press release regarding its financial results for the quarter ended September 30, 2025. A copy of the press release is furnished herewith as Exhibit 99.1 and is incorporated by reference herein. Neither the information included or incorporated by reference under this Item 2.02, nor the press release furnished herewith, shall be deemed to be “filed” for purposes of Sec…

Other Events. On October 29, 2025, the Company also announced that the Board of Directors of the Company declared, on October 28, 2025, a dividend of $0.47 per share, that is payable November 20, 2025 to shareholders of record of the Company as of November 10, 2025.

of this Current Report on Form 8-K, including Exhibit 99.1, shall neither be deemed filed for purposes of the Securities Exchange Act of 1934, as amended, nor shall such information or exhibit be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, except as expressly set forth by specific reference in such a filing. Cautionary Note Regarding Forward-looking Statements This Current Report on Form 8-K contains “forward-looking statements” within the mean…

Regulation FD Disclosure. On October 10, 2025, First Interstate BancSystem, Inc.’s wholly owned subsidiary, First Interstate Bank (“First Interstate”), closed the previously-announced transaction with Enterprise Bank & Trust (“Enterprise Bank”), a wholly owned subsidiary of Enterprise Financial Services Corp, pursuant to which Enterprise Bank acquired the twelve branches previously identified in Arizona and Kansas (the "Branches") from First Interstate, including specified deposits and loans,…

Chief Banking Officer — Lorrie Asker: The filing describes an internal realignment where the Chief Banking Officer role was split into two co-positions, with the incumbent retaining a senior role rather than departing.

Regulation FD Disclosure. On September 3, 2025, the Company appointed Mr. Shepler accepted the Company’s appointment as Co-Chief Banking Officer for the Company effective as of September 10, 2025. Mr. Shepler has more than 30 years of leadership experience in commercial, business, wealth, and consumer/small business banking, and he most recently served as Managing Director and National Commercial Banking Region Coverage Executive at Wells Fargo. In his role as Co-Chief Banking Officer, Mr. Sh…

Director — Renu Agrawal: The filing discloses the appointment of a new independent director to fill a vacancy created by a board size increase, which is a routine governance action rather than an executive departure.

Director — Mr. Scudder: The filing discloses the appointment of a director to specific board committees, which is a routine governance action rather than an executive departure.

Other Events. On August 28, 2025, the board of directors of First Interstate BancSystem, Inc. (the “Company”) adopted a new stock repurchase program, pursuant to which the Company has been authorized to repurchase up to $150 million worth of its issued and outstanding shares of common stock on or prior to March 31, 2027, which is the expiration date of the program. Any repurchased shares will be returned to authorized but unissued shares of common stock, as permitted under applicable Delaware…

Termination of a Material Definitive Agreement The information set forth in

Director — Michael L. Scudder: The filing discloses the appointment of a new independent director to fill a vacancy created by a board size increase, which is a routine governance action rather than an executive departure.

Other Events. On July 29, 2025, the Company also announced that the Board of Directors of the Company declared, on July 28, 2025, a dividend of $0.47 per share, that is payable August 21, 2025 to shareholders of record of the Company as of August 11, 2025.

Results of Operations and Financial Condition. On July 29, 2025, First Interstate BancSystem, Inc. (the “Company”) issued a press release regarding its financial results for the quarter ended June 30, 2025. A copy of the press release is furnished herewith as Exhibit 99.1 and is incorporated by reference herein. Neither the information included or incorporated by reference under this Item 2.02, nor the press release furnished herewith, shall be deemed to be “filed” for purposes of Section 18…