UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 6-K

Report of Foreign Private Issuer

Pursuant to Rule 13a-16 or 15d-16 under the

Securities Exchange Act of 1934

For the month of August 2026

Commission File Number: 001-39115

WISEKEY INTERNATIONAL HOLDING AG

(Exact Name of Registrant as Specified in Charter)

WISEKEY INTERNATIONAL HOLDING LTD

(Translation of Registrant’s name into English)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

☒ Form 20-F         ☐ Form 40-F

On August 6, 2026, SEALSQ Corp. (“SEALSQ”), a subsidiary of WISeKey International Holding Ltd. (“WISeKey”) entered into a subscription agreement (the “Subscription Agreement”) with WISeSat.Space Holdings Corp., a British Virgin Islands business company (“Pubco”) and Columbus Acquisition Corp., a Cayman Islands exempted company (“CAC”). The Subscription Agreement was entered into in connection with the proposed business combination among CAC, WISeSat.Space Corp., a British Virgin Islands business company d/b/a SpaceAIQ (the “SpaceAIQ”), and Pubco, pursuant to and in accordance with that certain Business Combination Agreement, dated as of November 9, 2025 (as amended, restated and/or supplemented from time to time in accordance with its terms, the “BCA”), by and among CAC, Pubco, WISeSat Merger Sub Corp., a Cayman Islands exempted company and a wholly owned subsidiary of Pubco (“Merger Sub”), SpaceAIQ and WISeKey, and to which SEALSQ (and together with WISeKey, the “Sellers”) became a party thereto as a Seller thereunder pursuant to a Joinder Agreement, dated as of December 12, 2025.

Pursuant to the Subscription Agreement, SEALSQ agreed to subscribe for and purchase, and Pubco agreed to issue and sell to SEALSQ, contemporaneously with the closing of the BCA, $10,000,000 in ordinary shares, no par value, of Pubco (“Pubco Ordinary Shares” and such shares issued hereunder, the “Subscription Shares”), at a price per share equal to the redemption price per share paid to Public Shareholders that redeem their CAC ordinary shares, par value $0.0001 per share (the “CAC Ordinary Shares”), in connection with the BCA, on the terms and subject to the conditions set forth in the Subscription Agreement. Assuming a redemption price of approximately $10.66 per share as of June 30, 2026, the number of Subscription Shares would be 938,086 Pubco Ordinary Shares.

On August 6, 2026, the parties to the BCA entered into the First Amendment (the “First Amendment”) to the BCA. Capitalized terms used herein but not defined herein have the meanings ascribed thereto in the BCA. Pursuant to the First Amendment, the parties agreed to extend the Outside Date to October 31, 2026.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.