Exhibit 5.1

October 31, 2013

VIA ELECTRONIC TRANSMISSION

Securities and Exchange Commission

100 F Street, N.E.

Washington, DC 20549

Re: Sorrento Therapeutics, Inc., Form S-3 Registration Statement

Ladies and Gentlemen:

We refer to the above-captioned registration statement on Form S-3 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Act”), filed by Sorrento Therapeutics, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission.

We have examined the originals, photocopies, certified copies or other evidence of such records of the Company, certificates of officers of the Company and public officials, and other documents as we have deemed relevant and necessary as a basis for the opinion hereinafter expressed. In such examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as certified copies or photocopies and the authenticity of the originals of such latter documents.

Based on our examination mentioned above, we are of the opinion that the securities being sold pursuant to the Registration Statement are duly authorized, legally and validly issued, fully paid and non-assessable.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement and to the reference to our firm under “Legal Matters” in the related Prospectus. In giving the foregoing consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Act, or the rules and regulations of the Securities and Exchange Commission.

Very truly yours,

/s/ Sichenzia Ross Friedman Ference LLP

Sichenzia Ross Friedman Ference LLP

61 Broadway    New York, New York 10006    212-930-9700    212-930-9725 Fax

www.srff.com