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NORTH POINT • 901 LAKESIDE AVENUE • CLEVELAND, OHIO 44114.1190 TELEPHONE: +1.216.586.3939 • JONESDAY.COM AMSTERDAM • ATLANTA • BEIJING • BOSTON • BRISBANE • BRUSSELS • CHICAGO • CLEVELAND • COLUMBUS • DALLAS DETROIT • DUBAI • DÜSSELDORF • FRANKFURT • HONG KONG • HOUSTON • IRVINE • LONDON • LOS ANGELES • MADRID MELBOURNE • MEXICO CITY • MIAMI • MILAN • MINNEAPOLIS • MUNICH • NEW YORK • PARIS • PERTH • PITTSBURGH SAN DIEGO • SAN FRANCISCO • SÃO PAULO • SHANGHAI • SILICON VALLEY • SINGAPORE • SYDNEY • TAIPEI • TOKYO • WASHINGTON October 31, 2025 Cleveland-Cliffs Inc. 200 Public Square, Suite 3300 Cleveland, Ohio 44114-2315 Re: 86,250,000 Common Shares of Cleveland-Cliffs Inc. Ladies and Gentlemen: We are acting as counsel for Cleveland-Cliffs Inc., an Ohio corporation (the “Company”), in connection with the public offering and sale of up to 86,250,000 shares (the “Shares”) of the Company’s common shares, par value $0.125 per share, by the Company pursuant to the Underwriting Agreement, dated as of October 29, 2025 (the “Underwriting Agreement”), by and between the Company and UBS Securities LLC, as the sole underwriter. In connection with the opinion expressed herein, we have examined such documents, records and matters of law as we have deemed relevant or necessary for purposes of such opinion. Based on the foregoing, and subject to the further limitations, qualifications and assumptions set forth herein, we are of the opinion that the Shares, when issued and delivered pursuant to the Underwriting Agreement against payment of the consideration therefor as provided in the Underwriting Agreement, will be validly issued, fully paid and nonassessable. As to facts material to the opinion and assumptions expressed herein, we have relied upon oral or written statements and representations of officers and other representatives of the Company and others. The opinion expressed herein is limited to the laws of the State of Ohio, as currently in effect, and we express no opinion as to the effect of the laws of any other jurisdiction on the opinion expressed herein. We hereby consent to the filing of this opinion as Exhibit 5.1 to the Current Report on Form 8-K dated the date hereof filed by the Company relating to the Company’s Registration Statement on Form S- 3 (Registration No. 333-291146) and to the reference to Jones Day under the caption “Legal Matters” in the prospectus supplement constituting a part of such Registration Statement. In giving such consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Securities Act of 1933 or the rules and regulations of the Securities and Exchange Commission promulgated thereunder. Very truly yours, /s/ Jones Day EXHIBIT 5.1