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): December 14, 2015

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:

ITEM 8.01 Other Events.

On December 14, 2015, Ameren Illinois Company (“Ameren Illinois”), a subsidiary of Ameren Corporation (“Ameren”), issued and sold $250,000,000 principal amount of its 4.15% Senior Secured Notes due 2046 (the “Notes”), pursuant to a Registration Statement on Form S-3 (File No. 333-205139-02), which became effective on June 22, 2015, and a Prospectus Supplement dated December 7, 2015 to a Prospectus dated June 22, 2015. Ameren Illinois received net offering proceeds of approximately $245.3 million, before expenses, upon closing of the transaction. Ameren Illinois intends to use the net offering proceeds to repay a portion of its short-term debt.

This Current Report on Form 8-K is being filed to report as exhibits certain documents in connection with the offering of the Notes.

ITEM 9.01 Financial Statements and Exhibits.

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.

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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.

Date: December 14, 2015

Exhibit Index