San Diego Web filtering software maker Websense Inc. signed a definitive agreement to be acquired by Vista Equity Partners, a national private equity firm based in Austin, Texas, for $25.75 per share in cash or an aggregate price of about $1 billion, the companies said.

The agreed upon price was 29 percent above Websense’s May 17 closing price, and a 53 percent premium to the stock’s average closing price over the past 60 days. Shares of Websense, traded on Nasdaq under the ticker WBSN, soared more than 28 percent in early trading May 20 to just below the agreed upon offer.

Websense Chairman John Carrington said the board approved the agreement to sell the business to Vista after holding detailed discussions with several potential buyers.

CEO John McCormick said Vista Equity shares a similar vision for the company including dedication to developing and delivering best in class cyber security to customers.

Founded in 1994 as NetPartners, the business that provides security protection software to companies grew rapidly in the late 1990s and earlier in the 2000s, but has slowed in recent years. Last year, it reported revenue of $361.5 million, down from $364.2 million in 2011.

Shares of Websense were just below $20 about a year earlier yet fell to about $13 in November before they began rising again.

The transaction is expected to close before the end of the third quarter. Upon closing, Websense will become a privately-held company of Vista, which has invested about $7 billion in technology-based companies. Websense senior management is expected to continue with the company and its headquarters are expected to remain in San Diego. The company has about 1,600 employees worldwide including about 500 locally.

— Mike Allen