Exhibit 8.1

August 27, 2015

EQT Midstream Partners, LP

625 Liberty Avenue, Suite 1700

Pittsburgh, Pennsylvania 15222

Ladies and Gentlemen:

We have acted as counsel to EQT Midstream Partners, LP, a Delaware limited partnership (the “Partnership”), in connection with the proposed offering and sale from time to time by the Partnership of common units representing limited partner interests in the Partnership having an aggregate offering price of up to $750,000,000 pursuant to that certain Equity Distribution Agreement dated August 27, 2015 (the “Distribution Agreement”) by and among the Partnership, on the one hand, and the several managers named therein, on the other.

In connection with this opinion, we have examined and relied upon the accuracy of original, certified copies or photocopies of such records, agreements, certificates and other documents as we have deemed necessary or appropriate to enable us to render the opinions set out below, including (i) the registration statement on Form S-3 (Registration No. 333-205812) (the “Registration Statement”) filed by the Partnership with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), which Registration Statement was declared effective by the Commission on August 18, 2015; (ii) the prospectus included in the Registration Statement dated August 18, 2015 (the “Base Prospectus”); (iii) the prospectus supplement to the Base Prospectus dated August 27, 2015 (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”); (iv) the Distribution Agreement and (v) the Partnership’s records and documents, certificates of representatives of the Partnership and public officials, and other instruments and documents as we deemed necessary or advisable for the purposes of this opinion.

In connection with the foregoing, we prepared (i) the discussion set forth under the caption “Material Income Tax Consequences” in the Prospectus Supplement and (ii) the discussion set forth under the caption “Material Income Tax Consequences” in the Base Prospectus (collectively, the “Discussion”).

We hereby confirm that all statements of legal conclusions, but not statements of factual matters, contained in the Discussion constitute the opinion of Baker Botts L.L.P. with respect to the matters set forth therein as of the date of the Prospectus Supplement, subject to the assumptions, qualifications, and limitations set forth therein.

We hereby consent to the filing of this opinion as Exhibit 8.1 to the Partnership’s Current Report on Form 8-K dated on or about the date hereof, to the incorporation by reference of this opinion into the Registration Statement, and to the reference to our firm and this opinion in the Discussion and under the caption “Legal Matters” in the Prospectus. In giving this consent, we do not hereby 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 Commission thereunder.

The opinion expressed herein is given as of the date hereof and we undertake no obligations to supplement this opinion if any applicable law changes after such date or if we become aware of any facts that might change the opinion expressed herein after such date or for any other reason.

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