Exhibit 5.1

Karen E. Deschaine

+1 858 550 6088

kdeschaine@cooley.com

April 13, 2017

Tocagen Inc.

3030 Bunker Hill Street, Suite 230

San Diego, CA 92109

Ladies and Gentlemen:

We have represented Tocagen Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission, covering the offering of up to an aggregate of 4,829,573 shares of the Company’s Common Stock, $0.001 par value (the “Shares”), including (i) 1,452,028 shares (the “2009 Plan Shares”) reserved for issuance upon the exercise of options issued under the Company’s 2009 Equity Incentive Plan, as amended (the “2009 Plan”), (ii) 3,127,545 shares (the “2017 Plan Shares”) reserved for issuance pursuant to the Company’s 2017 Equity Incentive Plan (the “2017 Plan”), and (iii) 250,000 shares (the “ESPP Shares”) reserved for issuance pursuant to the Company’s 2017 Employee Stock Purchase Plan (the “ESPP,” and collectively with the 2009 Plan and the 2017 Plan, the “Plans”).

In connection with this opinion, we have examined and relied upon the Registration Statement and the related prospectuses, the Company’s Fifteenth Amended and Restated Certificate of Incorporation, as amended, and Bylaws, as currently in effect, its forms of Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws to be effective upon the closing of the Company’s initial public offering, the Plans and such other documents, records, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies thereof.

Our opinion is expressed only with respect to the federal laws of the United States of America and the General Corporation Law of the State of Delaware. We express no opinion as to whether the laws of any particular jurisdiction other than those identified above are applicable to the subject matter hereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the 2009 Plan Shares, when sold and issued in accordance with the 2009 Plan, the 2017 Plan Shares, when sold and issued in accordance with the 2017 Plan, and the ESPP Shares, when sold and issued in accordance with the ESPP, and in each case when sold and issued in accordance with the Registration Statement and related prospectuses, will be validly issued, fully paid, and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

4401 EASTGATE MALL, SAN DIEGO, CA 92121 T: (858) 550-6000 F: (858) 550-6420 WWW.COOLEY.COM

Tocagen Inc.

April 13, 2017

Page Two

We consent to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,
COOLEY LLP
By:	/s/ Karen E. Deschaine
	Karen E. Deschaine

4401 EASTGATE MALL, SAN DIEGO, CA 92121 T: (858) 550-6000 F: (858) 550-6420 WWW.COOLEY.COM