Company: AILIM
Filing Date: 2025-09-16
Form Type: 424B2
Source: 0001104659-25-090464
Chunk: 2

Company: Ameren Illinois Co
Filing Date: 2025-09-16
Form: 424B2
Chunk 2
---
| ​ | Description of Preferred Stock                             | ​ | ​ | ​ | ​ | 39 | ​ | ​ |
| ​ | Book-Entry System                                          | ​ | ​ | ​ | ​ | 43 | ​ | ​ |
| ​ | Selling Securityholders                                    | ​ | ​ | ​ | ​ | 45 | ​ | ​ |
| ​ | Plan of Distribution                                       | ​ | ​ | ​ | ​ | 46 | ​ | ​ |
| ​ | Legal Matters                                              | ​ | ​ | ​ | ​ | 47 | ​ | ​ |
| ​ | Experts                                                    | ​ | ​ | ​ | ​ | 47 | ​ | ​ |

This prospectus supplement, the accompanying prospectus and any free writing prospectus that we prepare or authorize contain and/or incorporate by reference information that you should consider when making your investment decision. We have not, and the underwriters and their affiliates and agents have not, authorized any other person to provide you with additional or different information. If anyone provides you with additional or different information, you should not rely on it. We are not, and the underwriters and their affiliates and agents are not, making an offer to sell the bonds in any jurisdiction where the offer or sale is not permitted. You should not assume that the information in this prospectus supplement or in the accompanying prospectus is accurate as of any date other than the date on the front of those documents or that the information incorporated by reference is accurate as of any date other than the date of the document incorporated by reference in this prospectus supplement or the accompanying prospectus. Our business, financial condition, results of operations and prospects may have changed since then. In this prospectus supplement, “Ameren Illinois,” “we,” “us” and “our” refer to Ameren Illinois Company, doing business as Ameren Illinois.

S-i

TABLE OF CONTENTS

### ABOUT THIS PROSPECTUS SUPPLEMENT
This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of the bonds we are offering. The second part, the accompanying prospectus, gives more general information, some of which may not apply to the bonds we are offering in this prospectus supplement. See “Description of First Mortgage Bonds and Mortgage Indenture” in the accompanying prospectus. In the event that information in this prospectus supplement is inconsistent with information in the accompanying prospectus, you should rely on the information in this prospectus supplement.

This prospectus supplement and the accompanying