Filed Pursuant to Rule 424(b)(3)

Registration No. 333-289552

PROSPECTUS SUPPLEMENT NO. 7

(TO PROSPECTUS DATED SEPTEMBER 19, 2025)

75,000 Common Shares

Offered by the Selling Shareholders

RUBICO INC.

This is a supplement (the “Prospectus Supplement”) to the prospectus, dated September 19, 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. 333-289552), 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 December 4, 2025 (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 8 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 December 8, 2025.

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 December 2025

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 ☒ Form 40-F ☐

INFORMATION CONTAINED IN THIS FORM 6-K REPORT

On December 4, 2025, the Registrant issued a press release, a copy of which is attached hereto as Exhibit 99.1.

SIGNATURES

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

	Rubico Inc.
Date: December 8, 2025	By: /s/ Nikolaos Papastratis
	Nikolaos Papastratis
	Chief Financial Officer

Exhibit 99.1

Rubico Inc. Announces Letter of Intent for Acquisition of Newbuilding Mega Yacht

ATHENS, GREECE, Dec. 4, 2025 (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 it has entered into a letter of intent (the “LOI”) for the potential acquisition from Top Ships Inc. of a newbuilding mega yacht, the M/Y Sanlorenzo 1150Exp, with expected delivery in the second quarter of 2027, or the Newbuilding Yacht. The Newbuilding Yacht has a length of 60 meters and a gross tonnage of 1,150 tons with 6 guest cabins and is able to accommodate 12 guests and 15 crew.

The Company will make an advance cash payment of $4.0 million that will be credited against the acquisition price of the Newbuilding Yacht or refunded to the extent the Company does not elect to purchase the Newbuilding Yacht. The exclusivity period under the LOI will expire on March 31^st, 2026. Up to that date the Company will conduct a due diligence process and evaluation of the potential transaction taking into account market conditions and other factors, and there can be no assurance that potential transaction will be consummated. A special independent committee composed of independent members of the Company's board of directors will negotiate and approve any potential acquisition pursuant to the LOI.

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: npapastratis@rubicoinc.com

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 acquisition of the Newbuilding Yacht.

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.