Exhibit 5.1

December 27, 2018

Roadrunner Transportation Systems, Inc.

1431 Opus Place, Suite 530

Downers Grove, Illinois 60515

Re: Registration Statement on Form S-1

Ladies and Gentlemen:

We have acted as counsel to Roadrunner Transportation Systems, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing of the Registration Statement on Form S-1 (as amended or supplemented, the “Registration Statement”) filed on the date hereof by the Company with the Securities and Exchange Commission (the “Commission”) in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), relating to the proposed resale from time to time by the selling stockholders named in the Registration Statement of up to 7,801,625 shares of the Company’s common stock, par value $0.01 (the “Common Shares”). We are furnishing this opinion letter pursuant to Item 601(b)(5) of Regulation S-K under the Securities Act.

In connection with our opinion below, we have examined the Amended and Restated Certificate of Incorporation of the Company, the Second Amended and Restated Bylaws of the Company, records of proceedings of the Board of Directors of the Company deemed by us to be relevant to this opinion letter, and the Registration Statement, including the prospectus (the “Prospectus”) contained therein. We also have made such further legal and factual examinations and investigations as we deemed necessary for purposes of expressing the opinion set forth herein. In our examination, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as original documents and the conformity to original documents of all documents submitted to us as certified, conformed, facsimile, electronic or photostatic copies.

As to certain factual matters relevant to this opinion letter, we have relied conclusively upon originals or copies, certified or otherwise identified to our satisfaction, of such other records, agreements, documents and instruments, including certificates or comparable documents of officers of the Company and of public officials, as we have deemed appropriate as a basis for the opinion hereinafter set forth. Except to the extent expressly set forth herein, we have made no independent investigations with regard to matters of fact, and, accordingly, we do not express any opinion as to matters that might have been disclosed by independent verification.

Our opinion set forth below is limited to the General Corporation Law of the State of Delaware (including the statutory provisions, all applicable provisions of the Delaware constitution and reported judicial decisions interpreting the foregoing), and we do not express any opinion herein concerning any other laws.

This opinion letter is provided for use solely in connection with the transactions contemplated by the Registration Statement and may not be used, circulated, quoted or otherwise relied upon for any other purpose without our express written consent. The only opinion rendered by us consists of those matters set forth in the paragraph below, and no opinion may be implied or inferred beyond the opinion expressly stated. Our opinion expressed herein is as of the date hereof, and we undertake no obligation to advise you of any changes in applicable law or any other matters that may come to our attention after the date hereof that may affect our opinion expressed herein.

Based upon the foregoing, it is our opinion that, subject to the qualifications and limitations stated herein, we are of the opinion that the Common Shares have been validly issued, fully paid, and are nonassessable.

We consent to the filing of this opinion letter as an exhibit to the Registration Statement and to the use of our name under the heading “Legal Matters” in the Prospectus constituting a part thereof. In giving such consent, 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 thereunder.

Very truly yours,

/s/ Greenberg Traurig, LLP