Exhibit 99.1 For Immediate Release First Interstate BancSystem, Inc. Reports Fourth Quarter Earnings Billings, MT - January 28, 2026 - First Interstate BancSystem, Inc. (NASDAQ: FIBK) (the "Company") today reported financial results for the fourth quarter of 2025. For the quarter, the Company reported net income of $108.8 million, or$1.08 per diluted share, which compares to net income of $71.4 million, or$0.69 per diluted share, for the third quarter of 2025 and net income of $52.1 million, or$0.50 per ...