UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Schedule 13G/A

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

Barracuda Networks, Inc.

(Name of Issuer)

Common Stock, par value $0.001

(Title of Class of Securities)

068323 104

(CUSIP Number)

December 31, 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)

¨ Rule 13d-1(c)

x 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).

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Securities and Exchange Commission (the “Commission”) on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

All percentages set forth on the cover pages to this Schedule 13G/A and elsewhere were calculated based on 52,474,672 shares outstanding as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

Barracuda Networks, Inc. (the “Company”).

The Company’s principal executive offices are at 3175 S. Winchester Boulevard, Campbell, California 95008.

This Schedule 13G/A is being jointly filed by each of the following persons pursuant to Rule 13d-1(k) promulgated by the Commission pursuant to Section 13 of the Act: Francisco Partners, L.P. (“FP I”), Francisco Partners Fund A, L.P. (“FPFA”), FP Annual Fund Investors, LLC (“FPAFI”), Francisco Partners III, L.P. (“FP III”), Francisco Partners Parallel Fund III, L.P. (“FPPF III”), Francisco Partners GP, LLC (“FP GP LLC”), Francisco Partners Management, LP (“FPM LP”), Francisco Partners GP III, L.P. (“FP GP III LP”), and Francisco Partners GP III Management, LLC (“FP GP III Mgmt”) (collectively, the “Reporting Persons”).

The Reporting Persons have entered into a Joint Filing Agreement, dated February 6, 2015, a copy of which is attached as Exhibit A to this Statement, pursuant to which the Reporting Persons agreed to file this Schedule 13G/A and any amendments thereto jointly in accordance with the provisions of Rule 13d-1(k)(1) under the Act.

The address of the principal business office of each of the Reporting Persons is c/o Francisco Partners, One Letterman Drive, Building C—Suite 410, San Francisco, California 94129.

FP I, FPFA, FP III, FPPF III, FPM LP and FP GP III LP are limited partnerships organized under the laws of the State of Delaware. FPAFI, FP GP LLC and FP GP III Mgmt are limited liability companies organized under the laws of the State of Delaware.

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

068323 104

Not applicable.

FP GP LLC is the general partner of FP I and FPFA. FPM LP is the manager of FPAFI. FP GP III LP is the general partner of FP III and FPPF III, and FP GP III Mgmt is the general partner of FP GP III LP. The managers of FP GP LLC have the power, acting by majority vote, to vote or dispose of the shares held directly by FP I and FPFA, the managers of FP GP III Mgmt have the power, acting by majority vote, to vote or dispose of the shares held directly by FP III and FPPF.

III, and the partners of FPM LP have the power, acting by majority vote, to vote or dispose of the shares held directly by FPAFI. As a result of these relationships, each of FP GP LLC, FP GP III LP, FP GP III Mgmt and FPM LP has shared voting and dispositive power with respect to the shares of Common Stock directly held by FP I, FPFA, FPAFI, FP III and FPPF III.

All percentages calculated in this Schedule 13G/A are based upon an aggregate of 52,474,672 shares of Common Stock outstanding as of December 31, 2013 as disclosed in Barracuda Network, Inc.’s Quarterly Report on Form 10-Q for the quarter ended November 30, 2014 filed with the Commission on January 12, 2015.

Pursuant to Rule 13d-4 of the Act, the Reporting Persons expressly declare that the filing of this Schedule 13G/A shall not be construed as an admission that any such person is, for the purposes of Section 13(d) and/or Section 13(g) of the Act or otherwise, the beneficial owner of any securities covered by this statement held by any other person. The Reporting Persons expressly disclaim that they have agreed to act as a group other than as described in this Schedule 13G/A.

Not applicable.

See responses to items 4(a) and 4(b) above.

Not applicable.

The Reporting Persons may be deemed to be a “group” for purposes of Section 13(d)(3) of the Act. The Reporting Persons expressly disclaim that they have agreed to act as a group other than as described in this statement.

Not applicable.

Not applicable.

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.

Date: February 6, 2015

EXHIBIT A

AGREEMENT REGARDING THE JOINT FILING OF SCHEDULE 13G

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

Date: February 6, 2015