Cover and table of contents

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

OF THE SECURITIES EXCHANGE ACT OF 1934

For the fiscal year ended December 31, 2025

OR

OF THE SECURITIES EXCHANGE ACT OF 1934

For the transition period from to

Commission file number 001-16189

NiSource Inc.

(Exact name of registrant as specified in its charter)

(614) 460-6000

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes þ No ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15(d) of the Act. Yes ¨ No þ

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes þ No ¨

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes þ No ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definition of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and "emerging growth company" in Rule 12-b-2 of the Exchange Act.

Large accelerated filer þ Accelerated Filer ¨ Emerging Growth Company ☐ Non-accelerated Filer ¨ Smaller Reporting Company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report. ☑

If securities are registered pursuant to Section 12(b) of the Act, indicate by check mark whether the financial statements of the registrants included in the filing reflect the correction of an error to previously issued financial statements. ☐

Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis of incentive-based compensation received by any of the registrant's executive officers during the relevant recovery period pursuant to §240. 10D-1(b).☐

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act). Yes ☐ No þ

The aggregate market value of the registrant's common stock, par value $0.01 per share (the "Common Stock") held by non-affiliates was approximately $18,966,136,571 based upon the June 30, 2025, closing price of $40.34 on the New York Stock Exchange.

There were 478,533,171 shares of Common Stock outstanding as of February 4, 2026.

Documents Incorporated by Reference

Part III of this report incorporates by reference specific portions of the Registrant’s Notice of Annual Meeting and Proxy Statement relating to the Annual Meeting of Stockholders to be held on May 11, 2026.

CONTENTS

Note regarding forward-looking statements

This Annual Report on Form 10-K contains "forward-looking statements," within the meaning of Section 27A of the Securities Act of 1933, as amended (the "Securities Act"), and Section 21E of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). These forward-looking statements include, but are not limited to, statements concerning our plans, strategies, objectives, expected performance, planned expenditures, recovery of expenditures through rates, stated on either a consolidated or segment basis, and any and all underlying assumptions and other statements that are not statements of historical fact. Expressions of future goals and expectations and similar expressions reflecting something other than historical fact, including "may," "will," "should," "could," "would," "aims," "seeks," "expects," "plans," "anticipates," "intends," "believes," "estimates," "predicts," "potential," "targets," "forecast," and "continue," are intended to identify forward-looking statements. All forward-looking statements are based on assumptions that management believes to be reasonable; however, there can be no assurance that actual results will not differ materially. Investors and prospective investors should understand that many factors impact whether any forward-looking statement contained herein will, or can be, realized. Any one of those factors could cause actual results to differ materially from those projected.

Factors that could cause actual results to differ materially from those projected in any forward-looking statement discussed in this Annual Report on Form 10-K include, among other things:

• our ability to execute our business plan or growth strategy, including utility infrastructure investments, or business opportunities;

• our ability to manage data center growth in our service territories;

• potential incidents and other operating risks associated with our business;

• our ability to work successfully with our JV partners;

• our ability to construct, develop and place into service the Contract Assets and any other generation or transmission assets we develop to support future data center contracts on time or at all and consistent with initial cost estimates, as well as the performance of such assets once constructed and placed into service;

• our ability to obtain the significant additional financing required to construct the Contract Assets and any other generation or transmission assets we develop to support future data center contracts on favorable terms, if at all;

• our ability to recover our investments and realize our expected return under the ADS Contract and any future data center contracts that we enter into;

• our ability to maintain our investment grade credit ratings as we finance and pursue our data center strategy, including our performance under the ADS Contract and any future data center contracts that we enter into;

• ADS’ performance under the ADS Contract and the performance of our customers under any future data center contracts;

• any decision by ADS to terminate or reduce the committed capacity under the ADS Contract or any decision by any customer under any future data center contract to terminate or reduce the committed capacity under the contract;

• potential changes in the MISO accreditation treatment of capacity resources;

• our ability to adapt to, and manage costs related to, advances in technology, including alternative energy sources and changes in related laws and regulations;

• our increased dependency on technology;

• impacts related to our aging infrastructure;

• our ability to obtain sufficient insurance coverage and whether such coverage will protect us against significant losses;

• the success of our electric generation strategy;

• construction risks and supply risks;

• fluctuations in demand from residential and commercial customers;

• fluctuations in the price of energy commodities and related transportation costs or an inability to obtain an adequate, reliable and cost-effective fuel supply to meet customer demand;

• our ability to attract, retain or re-skill a qualified, workforce and maintain good labor relations;

• our ability to manage new initiatives and organizational changes;

• the performance and quality of third-party suppliers and service providers;

• our ability to manage the financial and operational risks related to achieving our carbon emission reduction goals, including our Net Zero Goal (as defined below), including any future associated impact from business opportunities such as data center development as those opportunities evolve;

• regulation and the impact of regulatory rate reviews;

• our ability to obtain expected financial or regulatory outcomes;

• potential cybersecurity attacks or security breaches;

• increased requirements and costs related to cybersecurity;

• any damage to our reputation;

• the impacts of natural disasters, potential terrorist attacks or other catastrophic events;

• the physical impacts of climate change and the transition to a lower carbon future;

• our debt obligations;

• any changes to our credit ratings or the credit ratings of certain of our subsidiaries;

• adverse economic and capital market conditions, including increases in inflation or interest rates, recession, or changes in investor sentiment;

• the actions of activist stockholders;

• economic conditions in certain industries;

• the ability of customers and suppliers to fulfill their payment and contractual obligations;

• the ability of our subsidiaries to generate cash;

• pension funding obligations;

• potential impairments of goodwill;

• the outcome of legal and regulatory proceedings, investigations, incidents, claims and litigation;

• compliance with changes in, or new interpretations of applicable laws, regulations and tariffs;

• the cost of compliance with environmental laws and regulations and the costs of associated liabilities;

• changes in tax laws or the interpretation thereof; and

• other matters set forth in Part I. Item 1, "Business," Part I, Item 1A, "Risk Factors" and Part II, Item 7, "Management’s Discussion and Analysis of Financial Condition and Results of Operations," of this report, some of which risks are beyond our control.

In addition, the relative contributions to profitability by each business segment, and the assumptions underlying the forward-looking statements relating thereto, may change over time.

All forward-looking statements are expressly qualified in their entirety by the foregoing cautionary statements. We undertake no obligation to, and expressly disclaim any such obligation to, update or revise any forward-looking statement to reflect changed assumptions, the occurrence of anticipated or unanticipated events or changes to expected results over time or otherwise, except as required by law.

NIS****OURCE INC.

PART I