Filed Pursuant to Rule 424(b)(3)

Registration No. 333-288796

Registration No. 333-291884

Registration No. 333-293441

Registration No. 333-294944

Registration No. 333-297276

PROSPECTUS SUPPLEMENT NO. 20

(TO PROSPECTUS DATED SEPTEMBER 19, 2025)

Up to 15,000,000 Common Shares 

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 Nos. 333-288796, 333-291884, 333-293441, 333-294944 and 333-297276), 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 July 15, 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 July 15, 2026.

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 July 15, 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 July 15, 2026.
 
The information contained in this Report, except for the commentary of Kalliopi Ornithopoulou contained in Exhibit 99.1, is hereby incorporated by reference into the Registrant’s registration statement on Form F-3 (File No. 333-297207).

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 Acquisition of Additional Newbuilding MR Tanker and a 33% Increase of Potential Gross Revenue Backlog to About $305 Million

ATHENS, Greece, July 15, 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 it has entered into a share purchase agreement (the “SPA”) with Top Ships Inc. to purchase the shares of a company (the “SPV”) that is party to a shipbuilding contract with Guangzhou Shipyard International Company Limited and China Shipbuilding Trading Co., Ltd. for the construction of a 47,499 dwt chemical/product oil carrier (the “Newbuilding MR Tanker”). The Newbuilding MR Tanker is scheduled for delivery in the third quarter of 2029.

The aggregate purchase price for 100% of the shares of the SPV is approximately $6.25 million, payable in full at closing. The transaction is expected to close by September 30, 2026, subject to customary closing conditions.

The SPV has secured time charter employment for the vessel with a major oil trader, starting from its delivery and for a firm duration of seven years, with charterer’s option to extend for four additional years. The total potential gross revenue backlog from this contract, including optional years, is about $75.4 million.

The SPV has also entered into a sale and leaseback financing agreement with a major Chinese leasing company for an amount of 85% of the installment payments under the shipbuilding contract. The purchase price under the shipbuilding contract, payable in installments up to the delivery of the vessel, is $45.2 million out of which $6.8 million has already been paid. The financing bears an interest rate of Term SOFR plus a margin of 1.80%. Under the financing, following the delivery of the vessel, the Company will pay quarterly installments of $0.5 million over a period of 10 years with a balloon payment of $18.2 million payable together with the last installment. Top Ships Inc. and the Company will provide corporate guarantees in favor of the leasing company.

The acquisition was approved by a special committee composed of independent and disinterested members of the Company’s board of directors, which obtained a fairness opinion with respect to the consideration paid to acquire the SPV from an independent financial advisor.

Kalliopi Ornithopoulou, the Company’s President, Chairwoman & Chief Executive Officer, stated:

“This acquisition, consistent with our strategy of redeploying capital into our core tanker business, marks a significant milestone that expands our fleet and strengthens our contracted revenue base. As a result, our total potential gross revenue backlog from our two newbuilding MR tankers increases to approximately $151.0 million. Including contracted time charters for our operating fleet, total potential gross revenue backlog—including optional years—rises to approximately $304.6 million, underscoring the strength and visibility of our future cash flows.

Our 2025 financial results, published on March 23, 2026, further highlight the Company’s earnings potential, with net income of $2.6 million, total assets of $134.1 million and stockholders’ equity of $45.8 million.”

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. Furthermore, the Company owns one 47,499 dwt MR tanker newbuilding scheduled for delivery in the fourth quarter of 2029 and a 60-meter newbuilding megayacht scheduled for delivery in the second quarter of 2027.

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 statements regarding future revenues and cash flows and the potential acquisition of newbuildings.

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.