Filed Pursuant to Rule 424(b)(3)
Registration No. 333-290370
PROXY STATEMENT/PROSPECTUS SUPPLEMENT NO. 1
(to Proxy
Statement/Prospectus dated January 12, 2026)
PROXY STATEMENT FOR EXTRAORDINARY GENERAL MEETING IN LIEU OF AN ANNUAL MEETING OF
Churchill Capital Corp IX
PROSPECTUS FOR UP TO 201,517,063 SHARES OF CLASS A COMMON STOCK
OF
CHURCHILL CAPITAL
CORP IX
(TO BE RENAMED “PLUSAI HOLDINGS, INC.” FOLLOWING DOMESTICATION
IN THE STATE OF DELAWARE AND IN CONNECTION WITH THE
BUSINESS COMBINATION DESCRIBED IN THIS PROXY STATEMENT)
This Proxy Statement/Prospectus Supplement No. 1 (this “Supplement”) supplements the proxy statement/prospectus dated January 12,
2026 that was mailed by Churchill Capital Corp IX, a Cayman Islands exempted company (“CCIX”), to its shareholders on or about January 12, 2026 (the “Proxy Statement/Prospectus”), in connection with the
proposed business combination among CCIX, AL Merger Sub I, Inc., a Delaware corporation and direct, wholly owned Subsidiary of CCIX, AL Merger Sub II, LLC, a Delaware limited liability company and a direct, wholly owned Subsidiary of CCIX, and Plus
Automation, Inc., a Delaware corporation (“PlusAI”). CCIX and PlusAI jointly filed the Proxy Statement/Prospectus with the U.S. Securities and Exchange Commission (the “SEC”) as part of a registration statement
on Form S-4 (Registration No. 333-290370), which was declared effective on January 12, 2026 by the SEC. Capitalized terms used in this Supplement and not
otherwise defined herein have the respective meanings ascribed to them in the Proxy Statement/Prospectus.
The purpose of this Supplement is to update and
supplement the information contained in the Proxy Statement/Prospectus with information contained in the Current Report on Form 8-K (the “Current Report”) filed by CCIX with the SEC on
January 27, 2026. The Current Report is attached to, and forms a part of, this Supplement.
This Supplement modifies and supersedes, in part, the
information in the Proxy Statement/Prospectus and is not complete without, and may not be delivered or utilized except in combination with, the Proxy Statement/Prospectus, including any amendments or supplements thereto. Any information in the Proxy
Statement/Prospectus that is modified or superseded by the information in the Current Report shall not be deemed to constitute a part of the Proxy Statement/Prospectus except as modified or superseded by this Supplement. This Supplement should be
read in conjunction with the Proxy Statement/Prospectus, and if there is any inconsistency between the information in the Proxy Statement/Prospectus and this Supplement, you should rely on the information in this Supplement.
YOU SHOULD CAREFULLY CONSIDER THE MATTERS DISCUSSED UNDER “RISK FACTORS” IN THE PROXY STATEMENT/PROSPECTUS.
NEITHER THE SEC NOR ANY STATE SECURITIES REGULATORY AGENCY HAS APPROVED OR DISAPPROVED THE TRANSACTIONS DESCRIBED IN THE PROXY STATEMENT/PROSPECTUS OR THIS
SUPPLEMENT, PASSED UPON THE MERITS OR FAIRNESS OF THE BUSINESS COMBINATION OR RELATED TRANSACTIONS OR PASSED UPON THE ADEQUACY OR ACCURACY OF THE DISCLOSURE IN THE PROXY STATEMENT/PROSPECTUS OR THIS SUPPLEMENT. ANY REPRESENTATION TO THE CONTRARY
CONSTITUTES A CRIMINAL OFFENSE.
The date of this Supplement is January 27, 2026.