UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Schedule 13G

Information to be Included in Statements Filed Pursuant to Rules 13d-1(b), (c) and (d)

and Amendments Thereto Filed Pursuant to Rule 13d-2(b)

Under the Securities Exchange Act of 1934

(Amendment No.      )*

A10 Networks, Inc.

(Name of Issuer)

Common Stock, $0.00001 par value

(Title of Class of Securities)

002121101

(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).

A10 Networks, Inc.

3 West Plumeria Drive, San Jose, California 95134.

This Schedule 13G is being filed by Summit Partners, L.P., Summit Partners GE VIII, LLC, Summit Partners GE VIII, L.P., Summit Partners Growth Equity Fund VIII-A, L.P., Summit Partners Growth Equity Fund VIII-B, L.P., Summit Master Company, LLC, Summit Investors Management, LLC, Summit Investors I, LLC, Summit Investors I (UK), L.P. (individually, an “Entity” and collectively, the “Entities”), Martin J. Mannion and Peter Y. Chung.

Summit Partners, L.P. is the managing member of Summit Partners GE VIII, LLC, which is the general partner of Summit Partners GE VIII, L.P., which is the general partner of Summit Partners Growth Equity Fund VIII-A, L.P. and Summit Partners Growth Equity Fund VIII-B, L.P. Summit Master Company, LLC is the managing member of Summit Investors Management, LLC, which is the manager of Summit Investors I, LLC and the general partner of Summit Investors I (UK), L.P. Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its investment committee responsible for voting and investment decisions with respect to A10 Networks, Inc. Summit Partners, L.P., through a two-person investment committee responsible for voting and investment decisions with respect to A10 Networks, Inc., currently composed of Martin J. Mannion and Mr. Chung, has voting and dispositive authority over the shares held by each of the Entities.

The address of the principal business office of Summit Partners, L.P., Summit Partners GE VIII, LLC, Summit Partners GE VIII, L.P., Summit Partners Growth Equity Fund VIII-A, L.P., Summit Partners Growth Equity Fund VIII-B, L.P., Summit Master Company, LLC, Summit Investors Management, LLC, Summit Investors I, LLC, Summit Investors I (UK), L.P., Martin J. Mannion and Peter Y. Chung is c/o Summit Partners, 222 Berkeley Street, 18th Floor, Boston, Massachusetts 02116.

Each of Summit Partners, L.P., Summit Partners GE VIII, L.P., Summit Partners Growth Equity Fund VIII-A, L.P. and Summit Partners Growth Equity Fund VIII-B, L.P. is a limited partnership organized under the laws of the State of Delaware. Each of Summit Partners GE VIII, LLC, Summit Master Company, LLC, Summit Investors Management, LLC and Summit Investors I, LLC is a limited liability company organized under the laws of the State of Delaware. Summit Investors I (UK), L.P. is a Cayman exempted limited partnership. Each of Martin J. Mannion and Peter Y. Chung is a United States citizen.

Common Stock, par value $0.00001 per share (the “Common Stock”)

002121101

Not applicable.

As of December 31, 2014, Summit Partners Growth Equity Fund VIII-A, L.P. was the record owner of 6,873,136 shares of Common Stock. As of December 31, 2014, Summit Partners Growth Equity Fund VIII-B, L.P. was the record owner of 2,510,989 shares of Common Stock. As of December 31, 2014, Summit Investors I, LLC was the record owner of 40,186 shares of Common Stock. As of December 31, 2014, Summit Investors I (UK), L.P. was the record owner of 3,535 shares of Common Stock.

By virtue of the affiliate relationships among the Entities and by virtue of Mr. Mannion’s and Mr. Chung’s membership on the two-person investment committee of Summit Partners, L.P., each of Summit Partners, L.P., Summit Partners GE VIII, LLC, Summit Partners GE VIII, L.P., Summit Partners Growth Equity Fund VIII-A, L.P., Summit Partners Growth Equity Fund VIII-B, L.P., Summit Master Company, LLC, Summit Investors Management, LLC, Summit Investors I, LLC, Summit Investors I (UK), L.P., Martin J. Mannion and Peter Y. Chung may be deemed to own beneficially all of the 9,427,846 shares of Common Stock as of December 31, 2014.

Each of the reporting persons expressly disclaims beneficial ownership of any shares of A10 Networks, Inc., except for Summit Partners Growth Equity Fund VIII-A, L.P., Summit Partners Growth Equity Fund VIII-B, L.P., Summit Investors I, LLC and Summit Investors I (UK), L.P., in each case, for the shares which it holds of record as provided above.

Summit Partners, L.P.: 15.5%

Summit Partners GE VIII, LLC: 15.5%

Summit Partners GE VIII, L.P.: 15.5%

Summit Partners Growth Equity Fund VIII-A, L.P.: 15.5%

Summit Partners Growth Equity Fund VIII-B, L.P.: 15.5%

Summit Master Company, LLC: 15.5%

Summit Investors Management, LLC: 15.5%

Summit Investors I, LLC: 15.5%

Summit Investors I (UK), L.P.: 15.5%

Martin J. Mannion: 15.5%

Peter Y. Chung: 15.5%

The foregoing percentages are calculated using the 60,715,534 shares of Common Stock outstanding as disclosed in A10 Networks, Inc.’s quarterly report on Form 10-Q filed with the Securities and Exchange Commission on November 7, 2014.

0 shares for each reporting person

Summit Partners, L.P.: 9,427,846

Summit Partners GE VIII, LLC: 9,427,846

Summit Partners GE VIII, L.P.: 9,427,846

Summit Partners Growth Equity Fund VIII-A, L.P.: 9,427,846

Summit Partners Growth Equity Fund VIII-B, L.P.: 9,427,846

Summit Master Company, LLC: 9,427,846

Summit Investors Management, LLC: 9,427,846

Summit Investors I, LLC: 9,427,846

Summit Investors I (UK), L.P.: 9,427,846

Martin J. Mannion: 9,427,846

Peter Y. Chung: 9,427,846

0 shares for each reporting person

Summit Partners, L.P.: 9,427,846

Summit Partners GE VIII, LLC: 9,427,846

Summit Partners GE VIII, L.P.: 9,427,846

Summit Partners Growth Equity Fund VIII-A, L.P.: 9,427,846

Summit Partners Growth Equity Fund VIII-B, L.P.: 9,427,846

Summit Master Company, LLC: 9,427,846

Summit Investors Management, LLC: 9,427,846

Summit Investors I, LLC: 9,427,846

Summit Investors I (UK), L.P.: 9,427,846

Martin J. Mannion: 9,427,846

Peter Y. Chung: 9,427,846

Not Applicable.

Not Applicable.

Not Applicable.

Not Applicable. The reporting persons expressly disclaim membership in a “group” as used in Section 13d-1(b)(1)(ii)(K).

Not Applicable.

Not Applicable. This statement on Schedule 13G is not filed pursuant to Rule 13d-1(b) or Rule 13d-1(c).

SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, we certify that the information set forth in this statement is true, complete and correct. We also hereby agree to file this statement jointly pursuant to the Agreement listed on Exhibit 1 hereto.

Dated: February 4, 2015.

Exhibit 1

AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree that only one statement containing the information required by Schedule 13G need be filed with respect to the ownership by each of the undersigned of shares of A10 Networks, Inc.

This agreement may be executed in any number of counterparts, each of which shall be deemed an original.

EXECUTED this 4th day of February 2015.

Exhibit 2

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Robin W. Devereux his or her true and lawful attorney-in-fact and agent, with full power of substitution and resubstitution, for him or her and in his or her name, place and stead, in any and all capacities (until revoked in writing) to sign any and all instruments, certificates and documents required to be executed on behalf of himself or herself individually or on behalf of each of any affiliate of Summit Partners, L.P. that is not a portfolio company, including without limitation those entities listed on the attached Exhibit A, on matters relating to:

and to file the same, with all exhibits thereto, and any other documents in connection therewith, with, as applicable, (i) the Securities and Exchange Commission, and with any other entity when and if such is mandated by the Exchange Act or by the By-laws of the National Association of Securities Dealers, and/or (ii) the Internal Revenue Service, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary fully to all intents and purposes as he or she might or could do in person thereby ratifying and confirming all that said attorney-in-fact and agent, or his or her substitute or substitutes, may lawfully do or cause to be done by virtue hereof. Said attorney-in-fact is entitled to amend Exhibit A from time to time to reflect additional affiliates of Summit Partners, L.P. that are not portfolio companies.

The undersigned hereby acknowledges that this Power of Attorney supersedes, revokes and terminates any power of attorney executed by the undersigned prior to the date hereof for the purposes listed above.

[Exhibit A and Signature Pages Follow]

Power of Attorney

Exhibit A

Power of Attorney

Exhibit A

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 27 day of August, 2008.

On this 27 day of August, 2008, before me personally came Gregory M. Avis, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came John R. Carroll, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 18 day of August, 2008.

On this 18 day of August, 2008, before me personally came Peter Y. Chung, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 5 day of Sept., 2008.

On this 5th day of September, 2008, before me personally came Scott C. Collins, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Christopher J. Dean, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Bruce R. Evans, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 18 day of August, 2008.

On this 18 day of August, 2008, before me personally came Charles J. Fitzgerald, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 21 day of August, 2008.

On this 21 day of August, 2008, before me personally came Craig D. Frances, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Thomas H. Jennings, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 18 day of August, 2008.

On this 18 day of August, 2008, before me personally came Walter G. Kortschak, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 5th day of September, 2008.

On this 5th day of September, 2008, before me personally came Sotiris T. Lyritzis, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Martin J. Mannion, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 12 day of August, 2008.

On this 12 day of August, 2008, before me personally came Harrison B. Miller, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Kevin P. Mohan, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Thomas S. Roberts, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 10 day of September, 2008.

On this 10 day of September, 2008, before me personally came E. Roe Stamps, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came Joseph F. Trustey, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 15 day of September, 2008.

On this 15 day of September, 2008, before me personally came

Stephen G. Woodsum, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 9th day of December, 2010.

On this 9 day of December, 2010, before me personally came Joseph J. Kardwell, known to me to be the person described and who executed the foregoing instrument that he acknowledged and executed the same.