The $350 million sale includes the SuperSonics and the WNBA's Storm from coffee magnate Howard Schultz to an investment group led by Clay Bennett.
The sale, which required a three-quarters majority from the board of governors, becomes official on October 31.
"We appreciate the support of the Board of Governors and we are honored to become a member of the NBA," Bennett said.
"We are excited about owning one of the outstanding franchises in sports in one of America's greatest cities. We are looking forward with great anticipation to the 40th anniversary season of the Sonics."
The Sonics, a fixture in Seattle since 1967, and have been looking for a new arena for some time.
Former owner Schultz was unable to come to an agreement with city officials on the amount of public funding he wanted for a facility to replace KeyArena.
Refurbished through public funding in 1994, KeyArena is smaller than some of the NBA's newer venues and has a revenue sharing agreement with the city that is unfavorable to the team.
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Bennett, the chairman of investment group Dorchester Capital, said when the initial agreement was reached in July that he would do all he could to keep the team in the Seattle area, including the possibility of an outlying suburb.
"The Board of Governors felt overwhelmingly that Clay is the right person to lead these extraordinary franchises into the future," NBA commissioner David Stern said.
"I fully recognize Clay's commitment to keeping the Sonics and Storm in Seattle, and we will do everything we can to support those efforts."
Schultz, who owns Starbucks Coffee, bought the team from Barry Ackerley in 2001 and claimed to have lost 60 million dollars during his ownership.