Exhibit 5.1

Hogan Lovells US LLP Columbia Square 555 Thirteenth Street, NW Washington, DC 20004-1109 T +1 202 637 5600 F +1 202 637 5910 www.hoganlovells.com

August 3, 2020

Board of Directors

Reliance Steel & Aluminum Co.

350 South Grand Avenue, Suite 5100

Los Angeles, CA 90071

Ladies and Gentlemen:

We are acting as counsel to Reliance Steel & Aluminum Co., a Delaware corporation (the “Company”), in connection with the issuance pursuant to an Indenture dated as of the date hereof (the “Base Indenture”), between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”), as supplemented by the First Supplemental Indenture, dated as of the date hereof relating to the Notes (as defined below) (the “First Supplemental Indenture” and, together with the Base Indenture, the “Indenture”), between the Company and the Trustee, of $400 million aggregate principal amount of the Company’s 1.300% Senior Notes due 2025 (the “2025 Notes”) and $500 million aggregate principal amount of the Company’s 2.150% Senior Notes due 2030 (the “2030 Notes” and, together with the 2025 Notes, the “Notes”), and the sale of the Notes pursuant to an Underwriting Agreement, dated July 28, 2020 (the “Agreement”), among the Company and J.P. Morgan Securities LLC, BofA Securities, Inc. and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein, and pursuant to the Company’s automatic shelf registration statement on Form S-3 (File No. 333-240139) filed with the Securities and Exchange Commission on July 28, 2020 (the “Registration Statement”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed. In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including pdfs). As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on. This opinion letter is given, and all statements herein are made, in the context of the foregoing.

For purposes of this opinion letter, we have assumed that (i) the Trustee, upon execution of the Indenture, will have all requisite power and authority under all applicable laws, regulations and governing documents to execute, deliver and perform its obligations under the Indenture and has complied with all legal requirements pertaining to its status as such status relates to the Trustee’s right to enforce the Indenture against the Company, (ii) the Trustee has authorized and will duly execute and deliver the Indenture, (iii) the Trustee is validly existing and in good standing in all necessary jurisdictions, (iv) upon its execution, the Indenture will constitute a valid and binding obligation, enforceable against the Trustee in accordance with its terms, (v) there will be no mutual mistake of fact or misunderstanding or fraud, duress or undue influence in connection with the negotiation, execution or delivery of the Indenture, and the conduct of all parties, upon execution, to the Indenture has complied and will comply with any requirements of good faith, fair dealing and conscionability and (vi) there are, have been and will be no agreements or understandings among the parties, written or oral, and there is, has been and will be no usage of trade or course of prior dealing among the parties that would define, supplement or qualify the terms, upon its execution, of the Indenture. We also have assumed the validity and constitutionality of each relevant statute, rule, regulation and agency action covered by this opinion letter.

Hogan Lovells US LLP is a limited liability partnership registered in the District of Columbia. “Hogan Lovells” is an international legal practice that includes Hogan Lovells US LLP and Hogan Lovells International LLP, with offices in: Alicante Amsterdam Baltimore Beijing Birmingham Boston Brussels Colorado Springs Denver Dubai Dusseldorf Frankfurt Hamburg Hanoi Ho Chi Minh City Hong Kong Houston Johannesburg London Los Angeles Luxembourg Madrid Mexico City Miami Milan Minneapolis Monterrey Moscow Munich New York Northern Virginia Paris Perth Philadelphia Rome San Francisco São Paulo Shanghai Silicon Valley Singapore Sydney Tokyo Warsaw Washington, D.C. Associated Offices: Budapest Jakarta Riyadh Shanghai FTZ Ulaanbaatar Zagreb. Business Service Centers: Johannesburg Louisville. Legal Services Center: Berlin. For more information see www.hoganlovells.com

This opinion letter is based as to matters of law solely on the applicable provisions of the following, as currently in effect: (i) the Delaware General Corporation Law, as amended; and (ii) the laws of the State of New York (but not including any laws, statutes, ordinances, administrative decisions, rules or regulations of any political subdivision below the state level). We express no opinion herein as to any other laws, statutes, ordinances, rules or regulations (and in particular, we express no opinion as to any effect that such other laws, statutes, ordinances, rules or regulations may have on the opinions expressed herein).

Based upon, subject to and limited by the foregoing, we are of the opinion that the Notes have been duly authorized on behalf of the Company and that, following (i) receipt by the Company of the consideration for the Notes specified in the Agreement, and (ii) the due execution, authentication, issuance and delivery of the Notes pursuant to the terms of the Indenture, the Notes will constitute valid and binding obligations of the Company.

The opinion expressed above with respect to the valid and binding nature of obligations may be limited by bankruptcy, insolvency, reorganization, receivership, moratorium or other laws affecting creditors’ rights (including, without limitation, the effect of statutory and other law regarding fraudulent conveyances, fraudulent transfers and preferential transfers) and by the exercise of judicial discretion and the application of principles of equity, good faith, fair dealing, reasonableness, conscionability and materiality (regardless of whether the Notes are considered in a proceeding in equity or at law).

This opinion letter has been prepared for use in connection with the filing by the Company of a Current Report on Form 8-K on the date hereof, which Form 8-K will be incorporated by reference in the Registration Statement and speaks as of the date hereof. We assume no obligation to advise of any changes in the foregoing subsequent to the delivery of this opinion letter.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the above-described Form 8-K and to the reference to this firm under the caption “Legal Matters” in the Prospectus dated July 28, 2020 that forms a part of the Registration Statement. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Securities Act of 1933, as amended.

Very truly yours,

/s/ HOGAN LOVELLS US LLP

HOGAN LOVELLS US LLP