Exhibit 99.1 For Immediate Release First Interstate BancSystem, Inc. Reports First Quarter Earnings Billings, MT - April 24, 2024 - First Interstate BancSystem, Inc. (NASDAQ: FIBK) (the “Company”) today reported financial results for the first quarter of 2024. For the quarter, the Company reported net income of $58.4 million, or $0.57 per share, which compares to net income of $61.5 million, or $0.59 per share, for the fourth quarter of 2023 and net income of $56.3 million, or $0.54 per share, for the first ...