Exhibit 99.1 For Immediate Release First Interstate BancSystem, Inc. Reports Second Quarter Earnings Billings, MT - July 29, 2025 - First Interstate BancSystem, Inc. (NASDAQ: FIBK) (the "Company") today reported financial results for the second quarter of 2025. For the quarter, the Company reported net income of $71.7 million, or$0.69 per diluted share, which compares to net income of $50.2 million, or$0.49 per diluted share, for the first quarter of 2025 and net income of $60.0 million, or$0.58 per dilu ...