Exhibit 5.1

September 24, 2013

HSBC USA Inc.

424 Fifth Avenue

New York, New York 10018

Ladies and Gentlemen:

We have acted as special counsel to HSBC USA Inc., a Maryland corporation (the “Company”), in connection with its offering of $250,000,000 aggregate principal amount of the Company’s Floating Rate Senior Notes due 2018 (the “Floating Rate Notes”) and $750,000,000 aggregate principal amount of the Company’s 2.625% Senior Notes due 2018 (the “Fixed Rate Notes” and, together with the Floating Rate Notes, the “Securities”) pursuant to a registration statement on Form S-3 (No. 333-180289). Such registration statement, as amended as of its most recent effective date (September 17, 2013), insofar as it relates to the Securities (as determined for purposes of Rule 430B(f)(2) under the Securities Act of 1933, as amended (the “Securities Act”)), is herein called the “Registration Statement” and the prospectus, dated March 22, 2012, as supplemented by the prospectus supplement thereto, dated September 17, 2013 is herein called the “Prospectus”. The Securities were issued under an indenture dated as of March 31, 2009 as supplemented by the First Supplemental Indenture dated as of March 22, 2012 (together, the “Indenture”), in each case between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”).

In arriving at the opinion expressed below, we have reviewed the following documents:

HSBC USA, Inc., p. 2

In addition, we have reviewed the originals or copies certified or otherwise identified to our satisfaction of all such other documents, and we have made such investigations of law, as we have deemed appropriate as a basis for the opinion expressed below.

In rendering the opinion expressed below, we have assumed the authenticity of all documents submitted to us as originals and the conformity to the originals of all documents submitted to us as copies. In addition, we have assumed and have not verified the accuracy as to factual matters of each document we have reviewed.

Based on the foregoing, and subject to the further assumptions and qualifications set forth below, it is our opinion that the Securities have been validly issued by the Company and are the valid, binding and enforceable obligations of the Company, entitled to the benefits of the Indenture.

Insofar as the foregoing opinion relates to the validity, binding effect or enforceability of the Securities, such opinion is subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and to general principles of equity.

In giving the foregoing opinion, we have assumed the correctness, without independent investigation, as to matters relating to the law of the State of Maryland, of the opinion of Wilmer Cutler Pickering Hale and Dorr LLP, a copy of which is filed as Exhibit 5.2 to the Current Report on Form 8-K of HSBC USA Inc. dated September 24, 2013 (the “September 2013 8-K”), and our opinion is subject to all of the limitations and qualifications contained therein. Except insofar as we have assumed the correctness of matters relating to the law of the State of Maryland, our opinion is limited to the federal law of the United States of America and the law of the State of New York.

We hereby consent to the use of our name in the Prospectus under the heading “Legal Matters,” as counsel for the Company that has passed on the validity of the Securities and to the filing of this opinion as Exhibit 5.1 to the September 2013 8-K. In giving such consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Securities and Exchange Commission thereunder.

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HSBC USA, Inc., p. 3

The opinion expressed herein is rendered on and as of the date hereof, and we assume no obligation to advise you or any other person, or to make any investigations, as to any legal developments or factual matters arising subsequent to the date hereof that might affect the opinion expressed herein.

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