Exhibit 5.1

John H. Sellers

(650) 843-5070

jsellers@cooley.com

April 3, 2015

NeoPhotonics Corporation

2911 Zanker Road

San Jose, California 95134

Ladies and Gentlemen:

We have acted as counsel to NeoPhotonics Corporation, a Delaware corporation (the “Company”), and you have requested our opinion in connection with the filing of a Registration Statement on Form S-1, as amended from time to time (the “Registration Statement”), with the Securities and Exchange Commission, including a related prospectus filed with the Registration Statement (the “Base Prospectus”), covering a public offering by a selling stockholder of up to 4,972,905 shares (the “Shares”) of the Company’s common stock, par value $0.0025 (“Common Stock”), all of which are currently outstanding. The Shares are being registered for offering and sale from time to time pursuant to Rule 415 under the Securities Act.

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and related Base Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and the Company’s Amended and Restated Bylaws, each as currently in effect, and (c) 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. 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, as amended.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares have been validly issued and are fully paid and non-assessable.

3175 HANOVER STREET, PALO ALTO, CA 94304-1130 T: (650) 843-5000 F: (650) 849-7400 WWW.COOLEY.COM

NeoPhotonics Corporation

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We consent to the reference to our firm under the caption “Legal matters” in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

3175 HANOVER STREET, PALO ALTO, CA 94304-1130 T: (650) 843-5000 F: (650) 849-7400 WWW.COOLEY.COM