13D Filing: Viex Capital Advisors, LLC and A10 Networks Inc. (ATEN)

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.  )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

825 Third Avenue, 33rd Floor

New York, New York 10022

STEVE
WOLOSKY, 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)

January 23, 2018

(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.

** Includes 120,000 shares underlying call options currently exercisable
as further described in Item 6.

* Includes 180,000 shares underlying call options currently exercisable
as further described in Item 6.

* Includes 120,000 shares underlying call options currently exercisable
as further described in Item 6.

* Includes 180,000 shares underlying call options currently exercisable
as further described in Item 6.

* Includes 300,000 shares underlying call options currently exercisable
as further described in Item 6.

* Includes 300,000 shares underlying call options currently exercisable
as further described in Item 6.

The following constitutes
the Schedule 13D filed by the undersigned (the “Schedule 13D”).

This statement relates
to the Common Stock, $0.0001 par value (the “Shares”), of A10 Networks, Inc., a Delaware corporation (the “Issuer”).
The address of the principal executive offices of the Issuer is 3 West Plumeria Drive, San Jose, California 95134.

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 825 Third Avenue, 33^rd Floor, New York, New York
10022.

(c)       The
principal business of Series One and VSO II 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
VIEX Capital is serving as the investment manager to Series One and VSO II. The principal occupation of Mr. Singer is serving as
the managing member of each of VIEX GP, VSO GP II 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, VSO II, VIEX GP, VSO GP II and VIEX Capital is organized under the laws of the State of Delaware. Mr. Singer is
a citizen of the United States of America.

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 1,719,575 Shares beneficially owned by Series One is approximately $10,087,240,
including brokerage commissions. The aggregate purchase price of the 120,000 Shares underlying certain call options which are currently
exercisable and may be deemed to be beneficially owned by Series One is approximately $5,300, 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 1,967,808 Shares beneficially owned by VSO II is approximately $11,373,729,
including brokerage commissions. The aggregate purchase price of the 180,000 Shares underlying certain call options which are currently
exercisable and may be deemed to be beneficially owned by VSO II is approximately $7,950, including brokerage commissions.

The Reporting Persons
acquired the Shares because they believe the Shares are meaningfully undervalued. The Reporting Persons are cognizant of the Issuer’s
financial and share price underperformance despite positive financial attributes of its business – namely nearly 80 percent gross
margins and a strong competitive position. The Reporting Persons believe in order for stockholder value to be maximized, the Issuer
must pursue one of two paths: either make immediate improvements to its financial model to accelerate profitability, or consider
leveraging its unique market position in a consolidating industry to pursue a strategic transaction where the Issuer is acquired
by another company. The Reporting Persons note that other companies have rewarded their stockholders by leveraging financial and
technological synergies to drive greater scale by pursuing strategic alternatives and consolidating into larger corporations. In
the nine months ended September 30, 2017, the Issuer has generated over $134 million in gross profit, or 77 percent of revenue,
but has been unable to earn an operating profit because of excessive operating expenses that consume all of this gross profit.
The status quo is not acceptable and the Reporting Persons are mindful of the upcoming nominating window for the election of directors.
The Reporting Persons also believe that Chairman and CEO Lee Chen and other Section 16 officers should immediately terminate their
routine 10b5-1 sales which, in the Reporting Persons’ view, do not align to stockholder value creation.

No Reporting Person
has any present plan or proposal which would relate to or result in any of the matters set forth in subparagraphs (a) – (j) of
Item 4 of Schedule 13D except as set forth herein or such as would occur upon or in connection with completion of, or following,
any of the actions discussed herein. The Reporting Persons intend to review their investment in the Issuer on a continuing basis.
Depending on various factors including, without limitation, the Issuer’s financial position and investment strategy, the
price levels of the Shares, conditions in the securities markets and general economic and industry conditions, the Reporting Persons
may in the future take such actions with respect to their investment in the Issuer as they deem appropriate including, without
limitation, engaging in communications with management and the Board of the Issuer, engaging in discussions with stockholders of
the Issuer and others about the Issuer and the Reporting Persons’ investment, making proposals to the Issuer concerning changes
to the capitalization, ownership structure, board structure (including board composition) or operations of the Issuer, purchasing
additional Shares, selling some or all of their Shares, engaging in short selling of or any hedging or similar transaction with
respect to the Shares, or changing their intention with respect to any and all matters referred to in Item 4.

The aggregate percentage
of Shares reported owned by each Reporting Person is based upon 70,751,450 Shares outstanding, which is the total number of Shares
outstanding as of October 25, 2017 as reported in the Issuer’s Quarterly Report on Form 10-Q, filed with the Securities and
Exchange Commission on November 2, 2017.

Percentage: 2.4%

Percentage: Approximately
2.8%

Percentage: 2.4%

Percentage: Approximately
2.8%

Percentage: Approximately
5.2%

Percentage: Approximately
5.2%

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.

On January 29, 2018,
the Reporting Persons entered into a Joint Filing Agreement in which the Reporting Persons agreed to the joint filing on behalf
of each of them of statements on Schedule 13D with respect to the securities of the Issuer to the extent required by applicable
law. The Joint Filing Agreement is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

Series One has purchased
in over-the-counter market American-style call options referencing an aggregate of 120,000 Shares, which have an exercise price
of $2.50 and expire on August 17, 2018, as further detailed on Schedule A hereto, which is incorporated by reference herein.

Series One has sold
in over-the-counter market American-style put options referencing an aggregate of 120,000 Shares, which have an exercise price
of $7.50 and expire on August 17, 2018, 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 180,000 Shares, which have an exercise price
of $2.50 and expire on August 17, 2018, 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 180,000 Shares, which have an exercise price of
$7.50 and expire on August 17, 2018, as further detailed on Schedule A hereto, which is incorporated by reference herein.

Other than as described
herein, there are no contracts, arrangements, understandings or relationships among the Reporting Persons, or between the Reporting
Persons and any other person, with respect to the securities of the Issuer.

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: January 29, 2018

SCHEDULE A

Transactions in the Shares During
the Past Sixty Days

VIEX
opportunities fund, LP – Series one

VIEX
special opportunities fund ii, LP

¹
Represents American-style call option purchased in the over-the-counter market with an expiration date of August 17, 2018.

² Represents American-style put option sold in the over-the-counter
market with an expiration date of August 17, 2018.