UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )

GENOCEA BIOSCIENCES, INC.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

372427104

(CUSIP Number)

February 10, 2014

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act, but shall be subject to all other provisions of the Act (however, see the Notes).

Genocea Biosciences, Inc., a Delaware corporation (the “Issuer”).

Cambridge Discovery Park

100 Acorn Park Drive, 5th Floor

Cambridge, MA 02140

The foregoing persons are collectively referred to as the “Reporting Persons.”

This statement relates to Common Stock held for the account of Cycad. Judson and Glenn are the sole managers and directors of Cycad (in such capacity, together the “Cycad Directors”).

The address of the business office of Cycad, Judson and Glenn is:

1270 Coast Village Circle

Santa Barbara, CA 93108

Common stock, $0.001 par value per share (“Common Stock”)

372427104

As of the date of this statement:

Cycad may be deemed to beneficially own 876,494 shares of Common Stock. Judson and Glenn, as the Cycad Directors, have shared voting and investment power with respect to the shares held by Cycad and may be deemed to beneficially own the shares held by Cycad. Judson and Glenn disclaim beneficial ownership of the shares beneficially owned by Cycad except to the extent of their pecuniary interest therein.

5.1% of the Common Stock, which percentage was calculated based on 17,217,104 shares of Common Stock outstanding following the completion of the Issuer’s initial public offering, as reported in the Issuer’s prospectus filed with the Securities and Exchange Commission on February 5, 2014.

See Row 5 of cover page for each Reporting Person.

See Row 6 of cover page for each Reporting Person.

See Row 7 of cover page for each Reporting Person.

See Row 8 of cover page for each Reporting Person.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

EXHIBIT INDEX

EXHIBIT A

JOINT FILING AGREEMENT

The undersigned hereby agree that a single Schedule 13G (or any amendment thereto) relating to the Common Stock of Genocea Biosciences, Inc. shall be filed on behalf of each of the undersigned and that this Agreement shall be filed as an exhibit to such Schedule 13G.

EXHIBIT B

POWER OF ATTORNEY

Each of the undersigned, Cycad Group, LLC, K. Leonard Judson and Paul F. Glenn (each, the “Undersigned”), hereby constitutes and appoints K. Leonard Judson, David E. Lafitte and Ryan C. Wilkins, or any of them signing individually, the Undersigned’s true and lawful attorney-in-fact to:

1. Complete and execute Forms 3, 4 and 5, and Schedules 13D and 13G, and such other forms and documents, including any amendments to any of the foregoing, as such attorney-in-fact shall in his discretion determine to be required or advisable pursuant to Section 16(a) and Section 13(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and the rules and regulations promulgated thereunder, or any successor laws and regulations, as a consequence of, or in respect of, the Undersigned’s ownership, acquisition or disposition of securities of Genocea Biosciences, Inc., a Delaware corporation (the “Company”);

2. Do and perform any and all acts for and on the behalf of the Undersigned which may be necessary or desirable in order to complete and execute any such forms or schedules, including any amendments thereto, and timely file such forms and schedules, including any amendments thereto, with the Securities and Exchange Commission, any securities exchange, the Financial Industry Regulatory Authority or any similar regulatory authority; and

3. Take any other action of any type whatsoever in connection with the foregoing which, in the opinion of such attorney-in-fact, may be of benefit to, in the best interest of, or legally required by, the Undersigned, it being understood that the documents executed by such attorney-in-fact on behalf of the Undersigned pursuant to this Power of Attorney shall be in such form and shall contain such information as such attorney-in-fact may approve in such attorney-in-fact’s discretion.

The Undersigned hereby grants to each such attorney-in-fact full power and authority to do and perform any and every act and thing whatsoever requisite, necessary, or proper to be done in the exercise of any of the rights and powers herein granted, as fully to all intents and purposes as the Undersigned might or could do if personally present, with full power of substitution or revocation, hereby ratifying and confirming all that such attorney-in-fact, or such attorney-in-fact’s substitute or substitutes, shall lawfully do or cause to be done by virtue of this Power of Attorney and the rights and powers herein granted. The Undersigned acknowledges that the foregoing attorneys-in-fact, in serving in such capacity at the request of the Undersigned, are not assuming any of the Undersigned’s responsibilities to comply with Sections 16 and 13(d) of the Exchange Act.

The Undersigned agrees that each such attorney-in-fact may rely entirely on information furnished orally or in writing by the Undersigned to each such attorney-in-fact. The Undersigned also agrees to indemnify and hold harmless each such attorney-in-fact against any losses, claims, damages or liabilities that arise out of or are based on any untrue statement or omission of necessary facts in the information provided by the Undersigned to such attorney-in-fact for purposes of executing, delivering and filing Forms 3, 4 and 5, and Schedules 13D and 13G, including any amendments to any of the foregoing, and agrees to reimburse each such attorney-in-fact for any legal or other expenses incurred in connection with investigating or defending against any such loss, claim, damage, liability or action.

This Power of Attorney shall remain in full force and effect until the Undersigned is no longer required to file (or no longer voluntarily files) Forms 3, 4 or 5, or Schedule 13D or 13G, with respect to the Undersigned’s holdings of and transactions in securities issued by the Company, unless earlier revoked by the Undersigned in a signed writing delivered to the Company and the foregoing attorneys-in-fact.

[Signature Page Follows]

IN WITNESS WHEREOF, the undersigned has executed this Power of Attorney, or caused this Power of Attorney to be executed, as of this 20th day of February 2014.