Exhibit 5.1

SIMPSON THACHER & BARTLETT LLP

425 LEXINGTON AVENUE

NEW YORK, N.Y. 10017-3954

(212) 455-2000

FACSIMILE (212) 455-2502

July 15, 2015

Blue Buffalo Pet Products, Inc.

11 River Road

Wilton, Connecticut 06897

Ladies and Gentlemen:

We have acted as counsel to Blue Buffalo Pet Products, Inc., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-1 (File No. 333-204847) (as amended, the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to (1) the issuance by the Company of 35,934 shares of Common Stock, par value $0.01 per share (“Common Stock”), to certain non-management employees of the Company as described in the Registration Statement (the “Company Shares”), (2) the sale of up to 33,619,533 shares of Common Stock by certain selling stockholders identified in the Registration Statement (together with any additional shares of Common Stock that may be sold by such selling stockholders pursuant to Rule 462(b) as prescribed by the Commission pursuant to the Securities Act, the “Selling Stockholders Outstanding Shares”) and (3) the sale by certain selling stockholders identified in the Registration Statement of up to 286,753 shares of Common Stock to be issued by the Company to such selling stockholders upon the exercise of options issued and outstanding under the Amended and Restated 2012 Stock Purchase and Option Plan of the Company (the “Plan”) (together with any additional shares of Common Stock that may be issued by the Company and sold by such selling stockholders pursuant to Rule 462(b) as prescribed by the Commission pursuant to the Securities Act, the “Selling Stockholders Stock Option Shares”).

We have examined the Registration Statement and a form of the Amended and Restated Certificate of Incorporation of the Company (the “Amended Certificate”), which has been filed with the Commission as an exhibit to the Registration Statement. We also have examined the originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and have made such other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that:

1. When the Amended Certificate has been duly filed with the Secretary of State of the State of Delaware and the Company Shares are issued and delivered to such non-management employees of the Company, the Company Shares will be validly issued, fully paid and nonassessable.

2. The Selling Stockholders Outstanding Shares have been validly issued, and are fully paid and nonassessable.

3. (A) When the Amended Certificate has been duly filed with the Secretary of State of the State of Delaware and (B) upon issuance and delivery in accordance with the terms of the Plan and the options issued pursuant thereto, the Selling Stockholders Stock Option Shares will be validly issued, fully paid and nonassessable.

2

We do not express any opinion herein concerning any law other than the Delaware General Corporation Law. We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Legal Matters” in the Prospectus included in the Registration Statement.

3