RIG: Unregistered securities sale

TRANSOCEAN LTD_May 22, 2026 ​ UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 ​ FORM 8-K ​ CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (date of earliest event reported): May 26, 2026 ( May 22, 2026 ) ​ TRANSOCEAN LTD . (Exact name of Registrant as specified in its charter) ​ Switzerland ​ ​ ​ 001-38373 ​ ​ ​ 98-0599916 (State or other jurisdiction of ​ (Commission ​ (I.R.S. Employer incorporation or organization) ​ File Number) ​ Identification No.) ​ ​ ​ ​ Turmstrasse 30 ​ ​ ​ Steinhausen , Switzerland ​ CH-6312 ​ ​ ​ (Address of principal executive offices) ​ (zip code) ​ Registrant’s telephone number, including area code: + 41 ( 41 ) 749-0500 Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions: ☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) ​ ☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12) ​ ☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) ​ ☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) ​ ​ ​ Securities registered pursuant to Section 12(b) of the Act Title of each class Trading Symbol Name of each exchange on which registered: Shares , $0.10 par value RIG New York Stock Exchange Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter). ​ Emerging growth company ☐ ​ If an emerging growth company, indi