UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a) AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No. 3)¹

A10 Networks, Inc.

(Name of Issuer)

Common Stock, $0.00001 par value

(Title of Class of Securities)

002121101

(CUSIP Number)

ERIC SINGER

VIEX CAPITAL ADVISORS, LLC

150 East 52^nd Street, 3^rd Floor

New York, New York 10022

STEVE WOLOSKY, ESQ.

ELIZABETH GONZALEZ-SUSSMAN, ESQ.

OLSHAN FROME WOLOSKY LLP

1325 Avenue of the Americas

New York, New York 10019

(212) 451-2300

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

June 4, 2019

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box ¨.

Note:  Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits.  See § 240.13d-7 for other parties to whom copies are to be sent. 

1              The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on 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).

* This Series One is part of a series of VIEX Opportunities Fund, LP, a series limited partnership.

* This Series Two is part of a series of VIEX Opportunities Fund, LP, a Delaware series limited partnership.

** Includes 150,900 Shares underlying call options currently exercisable as further described in Item 6.

*Includes 155,900 Shares underlying call options currently exercisable as further described in Item 6.

*Includes 216,300 Shares underlying call options currently exercisable as further described in Item 6.

* Includes 150,900 Shares underlying call options currently exercisable as further described in Item 6.

*Includes 155,900 Shares underlying call options currently exercisable as further described in Item 6.

* Includes 216,300 Shares underlying call options currently exercisable as further described in Item 6.

* Includes 523,100 Shares underlying call options currently exercisable as further described in Item 6.

* Includes 523,100 Shares underlying call options currently exercisable as further described in Item 6.

The following constitutes Amendment No. 3 to the Schedule 13D filed by the undersigned (the “Amendment No. 3”). This Amendment No. 3 amends the Schedule 13D as specifically set forth herein.

Item 2 is hereby amended and restated to read as follows:

Each of the foregoing is referred to as a “Reporting Person” and collectively as the “Reporting Persons.” Each of the Reporting Persons is party to that certain Joint Filing Agreement as further described in Item 6. Accordingly, the Reporting Persons are hereby filing a joint Schedule 13D.

(b)       The address of the principal office of each of the Reporting Persons is 150 East 52^nd Street, 3^rd Floor, New York, New York 10022.

(c)       The principal business of Series One, Series Two, VSO II and VSO III is investing in securities. The principal business of VIEX GP is acting as the general partner of Series One. The principal business of VSO GP II is acting as the general partner of VSO II. The principal business of VSO GP III is acting as the general partner of VSO III. The principal business of VIEX Capital is serving as the investment manager to Series One, Series Two, VSO II and VSO III. The principal occupation of Mr. Singer is serving as the managing member of each of VIEX GP, VSO GP II, VSO GP III and VIEX Capital.

(d)       No Reporting Person has, during the last five years, been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e)       No Reporting Person has, during the last five years, been party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f)       Each of Series One, Series Two, VSO II, VSO III, VIEX GP, VSO GP II, VSO GP III and VIEX Capital is organized under the laws of the State of Delaware. Mr. Singer is a citizen of the United States of America.

Item 3 is hereby amended and restated to read as follows:

The Shares purchased by Series One were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business) in open market purchases, except as otherwise noted, as set forth in Schedule A, which is incorporated by reference herein. The aggregate purchase price of the 1,659,818 Shares beneficially owned by Series One is approximately $10,599,104, including brokerage commissions.

The Shares purchased by Series Two were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business) in open market purchases, except as otherwise noted, as set forth in Schedule A, which is incorporated by reference herein. The aggregate purchase price of the 332,269 Shares beneficially owned by Series Two is approximately $2,151,393, including brokerage commissions. The aggregate purchase price of the 150,900 Shares underlying certain call options which are currently exercisable and may be deemed to be beneficially owned by Series Two is approximately $605,109, including brokerage commissions.

The Shares purchased by VSO II were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business) in open market purchases, except as otherwise noted, as set forth in Schedule A, which is incorporated by reference herein. The aggregate purchase price of the 3,074,948 Shares beneficially owned by VSO II is approximately $20,197,415, including brokerage commissions. The aggregate purchase price of the 155,900 Shares underlying certain call options which are currently exercisable and may be deemed to be beneficially owned by VSO II is approximately $575,000, including brokerage commissions.

The Shares purchased by VSO III were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business) in open market purchases, except as otherwise noted, as set forth in Schedule A, which is incorporated by reference herein. The aggregate purchase price of the 271,354 Shares beneficially owned by VSO III is approximately $1,682,368, including brokerage commissions. The aggregate purchase price of the 216,300 Shares underlying certain call options which are currently exercisable and may be deemed to be beneficially owned by VSO III is approximately $788,445, including brokerage commissions.

Items 5(a)-(c) are hereby amended and restated to read as follows:

The aggregate percentage of Shares reported owned by each Reporting Person is based upon 75,634,696 Shares outstanding, which is the total number of Shares outstanding as of April 30, 2019 as reported in the Issuer’s Quarterly Report on Form 10-Q, filed with the Securities and Exchange Commission on May 8, 2019.

Percentage: Approximately 2.2%

Percentage: Less than 1%

Percentage: Approximately 4.1%

Percentage: Less than 1%

Percentage: Approximately 2.8%

Percentage: Approximately 4.1%

Percentage: Less than 1%

Percentage: Approximately 7.5%

Percentage: Approximately 7.5%

The filing of this Schedule 13D shall not be construed as an admission that the Reporting Persons are, for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, the beneficial owners of any of the Shares reported herein. Each Reporting Person disclaims beneficial ownership of such Shares except to the extent of his or its pecuniary interest therein.

Item 6 is hereby by amended to add the following:

Series Two has purchased in over-the-counter market American-style call options referencing an aggregate of 150,900 Shares, which have an exercise price of $2.50 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has purchased in over-the-counter market American-style call options referencing an aggregate of 150,000 Shares, which have an exercise price of $2.50 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has purchased in over-the-counter market American-style call options referencing an aggregate of 5,900 Shares, which have an exercise price of $5.00 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has purchased in over-the-counter market American-style call options referencing an aggregate of 5,800 Shares, which have an exercise price of $5.00 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has sold in over-the-counter market American-style put options referencing an aggregate of 111,700 Shares, which have an exercise price of $5.00 and expire on August 16, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has sold in over-the-counter market American-style put options referencing an aggregate of 1,422,900 Shares, which have an exercise price of $7.50 and expire on August 16, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO II has sold in over-the-counter market American-style put options referencing an aggregate of 200,000 Shares, which have an exercise price of $7.50 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO III has purchased in over-the-counter market American-style call options referencing an aggregate of 210,500 Shares, which have an exercise price of $2.50 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

VSO III has purchased in over-the-counter market American-style call options referencing an aggregate of 5,800 Shares, which have an exercise price of $5.00 and expire on November 15, 2019, as further detailed on Schedule A hereto, which is incorporated by reference herein.

Item 7 is hereby amended to add the following exhibit:

SIGNATURES

After reasonable inquiry and to the best of his knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: June 4, 2019

SCHEDULE A

Transactions in the Securities of the Issuer During the Past 60 Days

VIEX opportunities fund, LP – Series one

VIEX special opportunities fund ii, LP

VIEX Special Opportunities Fund III, LP

VIEX opportunities fund, LP – Series TWO

Exhibit 99.1

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k)(1)(iii) under the Securities Exchange Act of 1934, as amended, the persons named below agree to the joint filing on behalf of each of them of a Statement on Schedule 13D (including additional amendments thereto) with respect to the shares of Common Stock, $0.0001 par value, of A10 Networks, Inc., a Delaware corporation. This Joint Filing Agreement shall be filed as an Exhibit to such Statement.

Dated: June 4, 2019