Exhibit 5.1

[LETTERHEAD OF GIBSON, DUNN & CRUTCHER LLP]

Client: 90818-00007

April 8, 2016

Spectra Energy Corp

5400 Westheimer Court

Houston, Texas 77056

Re: Spectra Energy Corp

Registration Statement on Form S-3 (File No. 333-188226)

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-3, File No. 333-188226 (the “Registration Statement”), of Spectra Energy Corp, a Delaware corporation (the “Company”), filed with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), in connection with the offering by the Company of up to 16,100,000 shares of the Company’s common stock, par value $0.001 per share (the “Shares”). The Shares will be issued pursuant to that certain Underwriting Agreement dated as of April 4, 2016 (the “Underwriting Agreement”) between the Company and the Underwriter named therein.

In arriving at the opinion expressed below, we have examined originals, or copies certified or otherwise identified to our satisfaction as being true and complete copies of the originals, of such documents, records, certificates of officers of the Company and of public officials and other instruments as we have deemed necessary or advisable to enable us to render this opinion. In our examination, we have assumed without independent investigation the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to original documents of all documents submitted to us as copies.

Based upon the foregoing, and subject to the assumptions, exceptions, qualifications and limitations set forth herein, we are of the opinion that the Shares, when issued against payment therefor as set forth in the Registration Statement, will be validly issued, fully paid and non-assessable.

The opinion expressed above is subject to the following exceptions, qualifications, limitations and assumptions:

A. We render no opinion herein as to matters involving the laws of any jurisdiction other than the Delaware General Corporation Law (the “DGCL”). This opinion is limited to the effect of the current state of the DGCL and the facts as they currently exist. We assume no obligation to revise or supplement this opinion in the event of future changes in such laws or the interpretations thereof or such facts.

April 8, 2016

Page 2

We consent to the filing of this opinion as an exhibit to the Registration Statement, and we further consent to the use of our name under the caption “Legal Matters” in the Registration Statement and the prospectus that forms a part thereof. In giving these consents, we do not thereby 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 promulgated thereunder.

Very truly yours,

/s/ Gibson, Dunn & Crutcher LLP