Shares of the Naples, Fla., company at last check were up 2.8% to $32.37. The stock is trading 57% above its 52-week low above $20, set in mid-March, and 20% below its 52-week high above $39, which was set in early January.
Starboard and its subsidiaries purchased 3.7 million shares of ACI for more than $100 million, a regulatory filing with the U.S. Securities and Exchange Commission shows.
The New York hedge fund now owns 10.5 million shares of ACI Worldwide, which has a total market capitalization of $3.7 billion, the filing said.
Starboard said the shares were undervalued and "represented an attractive investment opportunity," according to the SEC filing.
Starboard began building its stake during the first quarter of the year and owned 2 million shares, or 1.77%, at the end of the second quarter, Reuters reported.
The investor reported a 1.8% stake in an SEC Form 13-F as of June 30.
In August, Starboard was seeking to raise $300 million through a special-purpose-acquisition, or blank-check, company, to look for a business or businesses to operate.
Newly incorporated Starboard Value Acquisition Corp. SVACU was to sell 30 million units at $10 apiece, according to an SEC filing.
SVAC will look for a "merger, capital-stock exchange, asset acquisition, stock purchase, reorganization or similar business combination with one or more businesses," the company had said in an earlier statement.