Company: TSLTF
Filing Date: 2025-12-12
Form Type: SUPPL
Source: 0001193125-25-317786
Chunk: 4

Company: TRANSALTA CORP
Filing Date: 2025-12-12
Form: SUPPL
Chunk 4
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 located Suite 1400, 1100 1st Street S.E., Calgary, Alberta, T2G 1B1. Joint Book-Running Managers

| RBC Capital Markets |     | CIBC Capital Markets |     | BofA Securities |     | Morgan Stanley |

Co-Managers

|                     |     | Scotiabank                 |     |                         |
| BMO Capital Markets |     | TD Securities              |     | National Bank of Canada 
 Capital Markets         |
| ATB Capital Markets |     | Desjardins Capital Markets |     | MUFG                    |
| J.P. Morgan         |     | Mizuho                     |     | Loop Capital Markets    |

IMPORTANT NOTICE ABOUT INFORMATION IN

THIS PROSPECTUS SUPPLEMENT AND THE PROSPECTUS

This document is divided into two parts. The first part is this Prospectus Supplement, which describes the specific terms of the Notes and
also adds to and updates certain information contained in the Prospectus and the documents incorporated by reference into the Prospectus. The second part, the Prospectus, gives more general information, some of which may not apply to the Notes.
Capitalized terms used in this Prospectus Supplement that are not defined herein have the meanings ascribed thereto in the Prospectus.

Except as set forth under “The Offering” and “Description of Notes” in this Prospectus Supplement or
under “Description of Debt Securities” in the Prospectus, and unless the context otherwise requires, all references in this Prospectus Supplement to “TransAlta”, the “Corporation”,
“we”, “us” and “our” mean TransAlta Corporation and its consolidated subsidiaries including any consolidated partnerships of which the Corporation or any of its subsidiaries are partners.

If the description of the Notes or any other information varies between this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus, you should rely on the information in this Prospectus Supplement.

You should rely only on the information contained in this Prospectus Supplement or contained in, or incorporated by reference into, the Prospectus or in any free writing prospectus we authorize and use in connection with the offering of the Notes. We have not, and the underwriters have not, authorized anyone to provide you with different or additional information. We are not, and the underwriters are not, making an offer to sell the Notes in any jurisdiction where the offer or sale is not permitted. You should assume that the information in this Prospectus Supplement and the