Company Update

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549       FORM  8-K       CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934   Date of report (Date of earliest event reported): September 26, 2025         Commission File Number   Exact Name of Registrant as Specified in Charter; State of Incorporation; Address and Telephone Number   IRS Employer Identification Number   1-14756     Ameren Corporation ( Missouri Corporation) 1901 Chouteau Avenue St. Louis , Missouri 63103 ( 314 ) 621-3222       43-1723446 1-3672   Ameren Illinois Company ( Illinois Corporation) 10 Richard Mark Way Collinsville , Illinois 62234 ( 618 ) 343-8150   37-0211380   Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrants under any of the following provisions: ¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) ¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12) ¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) ¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))   Securities registered pursuant to Section 12(b) of the Act:   Title of each class   Trading Symbol(s)   Name of each exchange on which registered Common Stock, $0.01 par value per share   AEE   New York Stock Exchange   Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).     Emerging growth company   Ameren Corporation ¨   Ameren Illinois 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.   Ameren Corporation ¨   Ameren Illinois Company ¨       Co-Registrant CIK 0000018654 Co-Registrant Amendment Flag false Co-Registrant Form Type 8-K Co-Registrant DocumentperiodEndDate 2025-09-26 Co-Registrant Written Commuunications false Co-Registrant Solicitating Materials false Co-Registrant PreCommencement Tender Offer false Co-Registrant PreCommencement Tender Offer false Co-Registrant Entity PreCommencement Issuer Tender Offer false         ITEM 8.01 Other Events.   On September 26, 2025, Ameren Illinois Company (“Ameren Illinois”), a subsidiary of Ameren Corporation, sold $350 million principal amount of its 5.625% First Mortgage Bonds due 2055 (the “Bonds”). These Bonds are a further issuance of the 5.625% First Mortgage Bonds due 2055 previously issued on March 3, 2025, in the principal amount of $350 million. The Bonds were offered pursuant to a Registration Statement on Form S-3 (File No. 333-274977-01), which became effective on October 13, 2023, and a Prospectus Supplement dated September 15, 2025, to a Prospectus dated October 13, 2023. Ameren Illinois received net offering proceeds of approximately $358.1 million, before expenses, upon closing of the transaction.   This Current Report on Form 8-K is being filed to report as exhibits certain documents in connection with the offering of the Bonds.   ITEM 9.01 Financial Statements and Exhibits.   (d) Exhibits   Exhibit Number Title     1.1 Underwriting Agreement relating to the Bonds, dated September 15, 2025, between Ameren Illinois and the several underwriters named therein, for whom KeyBanc Capital Markets Inc., TD Securities (USA) LLC and U.S. Bancorp Investments, Inc. are acting as representatives.     4.1* General Mortgage Indenture and Deed of Trust, dated as of November 1, 1992, from Ameren Illinois to The Bank of New York Mellon Trust Company, N.A., as successor trustee (1992 Form 10-K, Exhibit 4(cc), File No. 1-3004).     4.2* Supplemental Indenture, dated as of February 1, 2025, by and between Ameren Illinois and The Bank of New York Mellon Trust Company, N.A., as successor trustee, relating to the Bonds (March 3, 2025 Form 8-K, Exhibit 4.2, File No. 1-3672).     5.1 Opinion of Stephen C. Lee, Esq., Vice President, Interim General Counsel and Secretary of Ameren Illinois, regarding the legality of the Bonds (including consent).     5.2 Opinion of Morgan, Lewis & Bockius LLP regarding the legality of the Bonds (including consent).     104 Cover Page Interactive Data File (formatted as Inline XBRL).     * Incorporated by reference as indicated.       This combined Form 8-K is being filed separately by Ameren Corporation and Ameren Illinois Company (each a “registrant”). Information contained herein relating to any individual registrant has been filed by such registrant on its own behalf. No registrant makes any representation as to information relating to any other registrant.     SIGNATURES   Pursuant to the requirements of the Securities Exchange Act of 1934, each registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized. The signature for each undersigned company shall be deemed to relate only to matters having reference to such company or its subsidiaries.     AMEREN CORPORATION   (Registrant)         By: /s/ Michael L. Moehn   Name: Michael L. Moehn   Title: Senior Executive Vice President and Chief Financial Officer         AMEREN ILLINOIS COMPANY   (Registrant)         By: /s/ Leonard P. Singh   Name: Leonard P. Singh   Title: Chairman and President   Date: September 26, 2025