Collins will join the Nets for their game Sunday night in Los Angeles against the Lakers. The 35-year-old center revealed at the end of last season he is gay, but he was a free agent and had remained unsigned.
With a need for another big man, the Nets turned to the 7-foot Collins, who helped them reach two NBA Finals in the early 2000s.
"The decision to sign Jason was a basketball decision," general manager Billy King said in a statement. "We needed to increase our depth inside, and with his experience and size, we felt he was the right choice for a 10-day contract."
Collins has played 12 NBA seasons, including his first seven with the Nets, when they were in New Jersey and Jason Kidd was their point guard. Kidd is now the Nets' coach and Collins has been a teammate of several other current Nets.
"Jason told us that his goal was to earn another contract with an NBA team. Today, I want to commend him on achieving his goal. I know everyone in the NBA family is excited for him and proud that our league fosters an inclusive and respectful environment," Commissioner Adam Silver said.
The Nets worked out Collins during the All-Star break and met with him again Sunday, with his twin brother, Jarron, hinting that history would be made.
"Hope everyone is enjoying their Sunday. Today should be a pretty cool day!" Jarron Collins wrote on Twitter.
Jason Collins played 38 games last season with Boston and Washington and averaged 1.1 points and 1.6 rebounds in limited minutes. For his career, the 7-foot Collins averages 3.6 points and 3.8 rebounds.
The Nets had an opening for a big man after trading Reggie Evans along with Jason Terry to Sacramento on Wednesday for guard Marcus Thornton.
The news on Collins comes as Michael Sam, the SEC defensive player of the year from Missouri who recently revealed he is gay, is taking part in the NFL draft combine. Sam's on-field workouts in Indianapolis are scheduled for Monday.
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