Alliant Energy Corporation

Item 8.01 Other Events. On August 18, 2026, Interstate Power and Light Company (“IPL”), a subsidiary of Alliant Energy Corporation, entered into an Underwriting Agreement (the “Underwriting Agreement”) with Mizuho Securities USA LLC, PNC Capital Markets LLC, U.S. Bancorp Investments, Inc., and Wells Fargo Securities, LLC, as representatives of the several underwriters listed therein (the “Underwriters”), pursuant to which IPL agreed to sell, and the Underwriters agreed to purchase, subject to the terms and conditions set forth therein, $500 million aggregate principal amount of IPL’s 5.100% Senior Debentures due 2031 (the “Debentures”), in a public offering (the “Offering”). The Debentures are to be issued under an Indenture dated as of August 20, 2003, between IPL and The Bank of New York Mellon Trust Company, N.A., as successor trustee (the “Indenture”), pursuant to an Officer’s Certificate provided pursuant to the Indenture setting forth the terms of the Debentures, dated as of August 21, 2026 (the “Certificate”). The Offering is expected to close, subject to standard closing conditions, on August 21, 2026. The Debentures are registered under the Securities Act of 1933, as amended, pursuant to an automatic shelf registration statement on Form S-3 (Registration No. 333-276062-01) that IPL filed with the Securities and Exchange Commission (the “SEC”) on December 15, 2023 (the “Registration Statement”). The Registration Statement was supplemented by a Prospectus Supplement setting forth the terms of the Debentures that IPL filed with the SEC on August 18, 2026. This Current Report on Form 8-K is being filed for the purpose of filing exhibits to the Registration Statement relating to the public offering of the Debentures, and all such exhibits are hereby incorporated into the Registration Statement by reference. The Underwriting Agreement is filed as Exhibit 1.1 and the Certificate is filed as Exhibit 4.1 to this Form 8-K. On August 18, 2026, IPL issued a press release announcing that it had priced the Offering. The press release is filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference. Ashurst Perkins Coie US LLP, counsel to IPL, has issued an opinion to IPL, dated August 21, 2026, regarding the legality of the Debentures upon issuance thereof, and Simmons Perrine PLC, Iowa counsel to IPL, has issued an opinion to IPL, dated August 21, 2026, regarding certain legal matters relating to Iowa law. The opinions are filed as Exhibit 5.1 and Exhibit 5.2, respectively, hereto.