SEC Form 424B3 filed by Rubico Inc.

Filed Pursuant to Rule 424(b)(3)

Registration No. 333-290426

PROSPECTUS SUPPLEMENT NO. 12

(TO PROSPECTUS DATED NOVEMBER 4, 2025)

12,315,270 Units,

Each Unit Consisting of One Common Share or One Pre-funded Warrant to Purchase One Common Share

and One Class A Warrant to Purchase One Common Share

615,763 Representative Warrants

(and up to 12,315,270 Common Shares Underlying the Pre-funded Warrants, up to 49,261,080 Common Shares Underlying the Class A Warrants and up to 2,463,052 Common Shares Underlying the Representative Warrants)

RUBICO INC.

This is a supplement (the “Prospectus Supplement”) to the prospectus, dated November 4, 2025 (as supplemented or amended from time to time, the “Prospectus”) of Rubico Inc. (the “Company”), which forms a part of the Company’s Registration Statement on Form F-1 (Registration No. 333-290426), as amended from time to time.

This Prospectus Supplement is being filed to update and supplement the information included in the Prospectus with the information contained in the Company’s Report on Form 6-K, furnished to the U.S. Securities and Exchange Commission (the “Commission”) on March 2, 2026 (the “Form 6-K”). Accordingly, the Form 6-K is attached to this Prospectus Supplement. 

This Prospectus Supplement should be read in conjunction with, and delivered with, the Prospectus and is qualified by reference to the Prospectus except to the extent that the information in this Prospectus Supplement supersedes the information contained in the Prospectus.

This Prospectus Supplement is not complete without, and may not be delivered or utilized except in connection with, the Prospectus, including any amendments or supplements to it.

Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 12 of the Prospectus for a discussion of information that should be considered in connection with an investment in our securities.

Neither the Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is March 2, 2026.

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 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 March 2026

Commission File Number: 001-42684

Rubico Inc.
(Translation of registrant's name into English)

20 Iouliou Kaisara Str
19002 Paiania
Athens, Greece
(Address of principal executive office)

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 [ X ] Form 40-F [ ]

On March 2, 2026, the Registrant issued a press release, a copy of which is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

Exhibit 99.1. Press release dated March 2, 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.

EXHIBIT 99.1

Rubico Announces Management Estimate of Net Asset Value to $94.2 Million

ATHENS, Greece, March 02, 2026 (GLOBE NEWSWIRE) -- Rubico Inc. (Nasdaq: RUBI) (the “Company” or “Rubico”), a global provider of shipping transportation services specializing in the ownership of vessels, announced today that after taking into account the most recent charter free vessel value estimates from a third party international broker, debt outstanding and cash, management estimates the Company’s net asset value (“NAV”) as of December 31, 2025 to be $94.2 million. This translates into a NAV of $22.88 per common share (based on number of common shares currently outstanding) and $15.08 per common share on a fully diluted basis (assuming exercise of all the 2,128,854 outstanding warrants and prefunded warrants).

The Company’s CEO said:

“As per the latest market close, we are trading at a 94.4% discount to the Company’s current estimate of the fully-diluted NAV of the Company. The company’s fleet has an average age of about five years, consisting of state-of-the-art vessels equipped with the latest eco-friendly and fuel-efficient specifications and features. Our time charters with high quality customers have been generating positive cashflow and income and we expect that they will continue the same way until their fixed period expires in Q1 2031.”

About the Company

Rubico Inc. is a global provider of shipping transportation services specializing in the ownership of vessels. The Company is an international owner and operator of two modern, fuel efficient, eco 157,000 dwt Suezmax tankers.   

The Company is incorporated under the laws of the Republic of the Marshall Islands and has executive offices in Athens, Greece. The Company's common shares trade on the Nasdaq Capital Market under the symbol “RUBI”.
Please visit the Company’s website at: https://rubicoinc.com/

For further information please contact:
Nikolaos Papastratis
Chief Financial Officer
Rubico Inc.
Tel: +30 210 812 8107
Email: [email protected]

Forward-Looking Statements

Matters discussed in this press release may constitute forward-looking statements. The Private Securities Litigation Reform Act of 1995 provides safe harbor protections for forward-looking statements in order to encourage companies to provide prospective information about their business. Forward-looking statements include statements concerning plans, objectives, goals, strategies, future events or performance, and underlying assumptions and other statements, which are other than statements of historical facts, including with respect to the Company’s future operational performance and the trading of the Company’s shares.

The Company desires to take advantage of the safe harbor provisions of the Private Securities Litigation Reform Act of 1995 and is including this cautionary statement in connection with this safe harbor legislation. The words “believe,” “anticipate,” “intends,” “estimate,” “forecast,” “project,” “plan,” “potential,” “may,” “should,” “expect” “pending” and similar expressions identify forward-looking statements. The forward-looking statements in this press release are based upon various assumptions, many of which are based, in turn, upon further assumptions, including without limitation, our management's examination of historical operating trends, data contained in our records and other data available from third parties. Although we believe that these assumptions were reasonable when made, because these assumptions are inherently subject to significant uncertainties and contingencies which are difficult or impossible to predict and are beyond our control, we cannot assure you that we will achieve or accomplish these expectations, beliefs or projections. Please see the Company’s filings with the Securities and Exchange Commission for a more complete discussion of these and other risks and uncertainties. The information set forth herein speaks only as of the date hereof, and the Company disclaims any intention or obligation to update any forward-looking statements as a result of developments occurring after the date of this communication.