Exhibit 99.1 FOR IMMEDIATE RELEASE: February 21, 2024 NiSource announces fourth quarter 2023 results • Achieved 2023 non-GAAP NOEPS at top end of guidance range • Raising 2024 non-GAAP NOEPS guidance to $1.70-1.74 from$1.68-1.72 • Reaffirming 6-8% annual 2023-2028 non-GAAP NOEPS growth • Successfully closed NIPSCO minority interest transaction in December MERRILLVILLE, Ind. - NiSource Inc. (NYSE: NI) today announced, on a GAAP basis, 2023 net income available to common shareholders of $661.7 million, or$1 ...