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The information contained in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Filed Pursuant to Rule 424(b)(3)
Registration No. 333-297949

Subject to Completion
Preliminary Prospectus Supplement dated September 8, 2026

PRELIMINARY PROSPECTUS SUPPLEMENT
(To prospectus dated August 4, 2026)

Junior Subordinated Notes due 2057

Ameren Corporation is offering $ principal amount of its Junior Subordinated Notes due 2057, referred to in this prospectus supplement as the “junior subordinated notes.” The junior subordinated notes will mature on March 15, 2057. The junior subordinated notes will bear interest (i) from and including the date of original issuance to but excluding March 15, 2032 at an annual rate of % and (ii) from and including March 15, 2032 during each Interest Reset Period (as defined herein) at an annual rate equal to the Five-Year Treasury Rate (as defined herein) as of the most recent Reset Interest Determination Date (as defined herein) plus %; provided, that the interest rate during any Interest Reset Period will not reset below % (which equals the initial interest rate on the junior subordinated notes). We will pay interest on the junior subordinated notes semi-annually in arrears on March 15 and September 15 of each year. The first such payment will be made on March 15, 2027. The junior subordinated notes will be issued only in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. We may redeem at our option all or a portion of the junior subordinated notes as described in this prospectus supplement under “Description of Junior Subordinated Notes — Redemption.”

We may defer interest payments on the junior subordinated notes on one or more occasions for up to 10 consecutive years per deferral period as described in this prospectus supplement. Deferred interest payments will accrue additional interest at a rate equal to the interest rate then applicable to the junior subordinated notes, to the extent permitted by applicable law.

The junior subordinated notes will be our direct unsecured general obligations and will rank junior and be subordinated in right of payment and upon liquidation to our existing and future Senior Indebtedness (as defined herein) and equally in right of payment with any Pari Passu Securities (as defined herein).

The junior subordinated notes are a new issue of securities with no established trading market. We do not intend to apply for listing of the junior subordinated notes on any securities exchange.

This prospectus supplement and the accompanying prospectus are not intended to constitute an offer to, and the junior subordinated notes should not be purchased, held or otherwise acquired by, a “specified foreign entity”, as defined in Section 7701(a)(51)(B) of the Internal Revenue Code of 1986, as amended (an “SFE”). By purchasing the junior subordinated notes, any investor in the junior subordinated notes (including all affiliated entities that participate in such purchase) will be deemed to represent and warrant to us that it is not, and will not be, for its taxable year that includes the date of the original issuance of the junior subordinated notes, an SFE.

Investing in the junior subordinated notes involves risks. See “Risk Factors” on page S-6 of this prospectus supplement.

Neither the Securities and Exchange Commission nor any other regulatory body has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

Offering
Price(1)

Underwriting
Discount

Proceeds,
Before Expenses,
to Ameren Corporation

Per junior subordinated note

Total

(1)

Plus accrued interest from September , 2026 if settlement occurs after that date.

The underwriters expect to deliver the junior subordinated notes in book-entry only form through the facilities of The Depository Trust Company for the accounts of its participants, including Clearstream Banking, société anonyme, and Euroclear Bank SA/NV, as operator of the Euroclear System, against payment in New York, New York on or about September , 2026.

Joint Book-Running Managers

Barclays

BofA Securities

J.P. Morgan

Morgan Stanley

MUFG

Truist Securities

PNC Capital Markets LLC

Scotiabank

September , 2026

	Page
Prospectus Supplement
About This Prospectus Supplement		S-ii
Prospectus Supplement Summary		S-1
Risk Factors		S-6
Where You Can Find More Information		S-10
Capitalization		S-11
Use of Proceeds		S-11
Description of Junior Subordinated Notes		S-12
Material United States Federal Income Tax Consequences		S-23
Underwriting		S-29
Legal Matters		S-34
Prospectus
About this Prospectus		1
Ameren Corporation		1
Risk Factors		2
Where You Can Find More Information		3
Use of Proceeds		5
Description of Debt Securities		6
Description of Common Stock		15
Description of Preferred Stock		17
Description of Stock Purchase Contracts and Stock Purchase Units		19
Book-Entry System		20
Selling Securityholders		22
Plan of Distribution		23
Legal Matters		24
Experts		24

	As of June 30, 2026
	Amount			Percent of Total Capitalization
	(in millions)
Short-term debt(1)		$	1,220		3.4%
Long-term debt (including current maturities)(2)			20,758		58.0
Total short-term debt and long-term debt (including current maturities)			21,978		61.4
Noncontrolling interests			129		0.4
Total Ameren Corporation shareholders’ equity			13,687		38.2
Total capitalization		$	35,794		100.0%

Underwriter	Principal Amount
Barclays Capital Inc.		$
BofA Securities, Inc.
J.P. Morgan Securities LLC
Morgan Stanley & Co. LLC
MUFG Securities Americas Inc.
Truist Securities, Inc.
PNC Capital Markets LLC
Scotia Capital (USA) Inc.
Total		$