UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): April 21, 2020

ACNB Corporation

(Exact name of Registrant as specified in its charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the Registrant under any of the following provisions (see General Instruction A.2. below):

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Indicate by check mark whether the Registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ¨

If an emerging growth company, indicate by check mark if the Registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ¨

Securities registered pursuant to Section 12(b) of the Act:

CURRENT REPORT ON FORM 8-K

On April 21, 2020, the Board of Directors of ACNB Corporation (the “Corporation”) amended Article I, Section 101, of the Bylaws of the Corporation to clarify that the Board of Directors shall have the right to determine that a meeting of shareholders be held solely by means of the Internet or other electronic communications technology in the manner and to the extent provided by the Pennsylvania Business Corporation Law.

The foregoing description of the amendment to the Bylaws does not purport to be complete and is qualified in its entirety by reference to the full text of the Bylaws, which are attached hereto as Exhibit 3.2 and incorporated herein by reference.

(d) Exhibits.

EXHIBIT INDEX

EXHIBIT NO.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this Current Report on Form 8-K to be signed on its behalf by the undersigned, thereunto duly authorized.