Exhibit 5.1

Matthew T. Browne

(858) 550-6045

mbrowne@cooley.com

May 12, 2015

U.S. Auto Parts Network, Inc.

16941 Keegan Avenue

Carson, CA 90746

Ladies and Gentlemen:

You have requested our opinion, as counsel to U.S. Auto Parts Network, Inc., a Delaware corporation (the “Company”), with respect to certain matters in connection with the filing by the Company of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission, covering the offering of an aggregate of up to 1,500,000 shares of the Company’s Common Stock, $0.001 par value (the “Shares”), issuable pursuant to the Company’s 2007 Omnibus Incentive Plan (the “2007 Plan”).

In connection with this opinion, we have examined and relied upon the Registration Statement and related prospectus, the 2007 Plan, the Company’s Second Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, and the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies thereof (except that such assumption is not made with respect to the due execution and delivery of documents by the Company). As to certain factual matters, we have relied upon a certificate of officers of the Company and have not sought to independently verify such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion as to whether the laws of any particular jurisdiction other than those identified above are applicable to the subject matter of this opinion. We are not rendering any opinion as to compliance with any federal or state antifraud law, rule or regulation relating to securities, or to the sale or issuance thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when issued and sold in accordance with the 2007 Plan, the Registration Statement and related prospectus, will be validly issued, fully paid and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

We consent to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

4401 EASTGATE MALL, SAN DIEGO, CA 92121    T: (858) 550-6000    F: (858) 550-6420    WWW.COOLEY.COM