Company: RIG
Filing Date: 2025-09-24
Form Type: 424B5
Source: 0001451505-25-000097
Chunk: 0

Company: Transocean Ltd.
Filing Date: 2025-09-24
Form: 424B5
Chunk 0
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Filed Pursuant to Rule 424(b)(5)
Registration No. 333-280617 The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has become effective under the Securities Act of 1933, as amended. This preliminary prospectus supplement and the accompanying base prospectus are not offers to sell these securities, nor are they soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted. Subject to completion, dated September 24, 2025 Preliminary Prospectus Supplement
(To Prospectus dated June 28, 2024) Transocean Ltd. 100,000,000 Shares We are offering to sell 100,000,000 of our shares, par value $0.10 (“shares”). Our shares are listed on the New York Stock Exchange (“NYSE”) under the symbol “RIG.” The last reported sale price of our shares on the NYSE on September 22, 2025 was $3.39 per share.

| ​                                | ​ | Per Share |   | ​ | ​ | Total |   | ​ |
|:---------------------------------|:--|:----------|:--|:--|:--|:------|:--|:--|
| Price to the public              | ​ | $         | ​ | ​ | ​ | $     | ​ | ​ |
| Underwriting discount(1)         | ​ | $         | ​ | ​ | ​ | $     | ​ | ​ |
| Proceeds, before expenses, to us | ​ | $         | ​ | ​ | ​ | $     | ​ | ​ |

(1) Please read “Underwriting (Conflicts of Interest)” for additional information regarding underwriting compensation An affiliate of one of our directors, Frederik Mohn, has provided an indication of interest in purchasing up to an aggregate of 4,000,000 shares from the underwriters at the public offering price.The number of our shares available for sale to the public will be reduced to the extent such person purchases such shares. The underwriters will not receive any underwriting discounts or commissions on any shares sold to any affiliate of Frederik Mohn. The indication of interest is not a commitment to purchase, and such person is not obligated to purchase any of our shares in this offering. Please read “Underwriting” for additional information. We have granted the underwriters an option for a period of 30 days to purchase up to an additional