The 34-year-old center, who has played for six teams in 12 seasons, wrote a first-person account that was posted on Sports Illustrated's website Monday. Collins finished the season with the Washington Wizards and is now a free agent. He says he wants to keep playing.
"If I had my way, someone else would have already done this," he writes. "Nobody has, which is why I'm raising my hand."
Collins played in a Final Four for Stanford and reached two NBA Finals. His twin brother, Jarron, was also a longtime NBA center. Collins says he told his brother he was gay last summer.
"Jason has been a widely respected player and teammate throughout his career and we are proud he has assumed the leadership mantle on this very important issue," NBA commissioner David Stern said in a statement.
Among those offering support on social media was former first daughter Chelsea Clinton, a friend from Stanford.
She tweeted: "Very proud of my friend Jason Collins for having the strength & courage to be the first openly gay player in the NBA."
Collins was also college roommates with another member of an American political dynasty: Rep. Joe Kennedy III, D-Mass. In his account, Collins wrote that he realized he needed to go public when the congressman walked in Boston's gay pride parade last year -- and Collins couldn't join him.
Kennedy tweeted Monday that "I've always been proud to call (Collins) a friend, and I'm even prouder to stand with him today."
Mostly a backup in his career, Collins has averaged 3.6 points and 3.8 rebounds for the Nets, Grizzlies, Timberwolves, Hawks, Celtics and Wizards. He was traded from Boston to Washington in February. Collins was the 18th pick in the first round of the 2001 NBA draft.
Several male athletes have previously come out after they retired, including the NBA's John Amaechi, the NFL's Esera Tuaolo and Major League Baseball's Billy Bean. But Collins is the first to do so while planning to keep playing.
Advocacy organization GLAAD released a statement from Aaron McQuade, who head of its sports program.
"`Courage' and `inspiration' are words that get thrown around a lot in sports, but Jason Collins has given both ideas a brand new context," he said. "We hope that his future team will welcome him, and that fans of the NBA and sports in general will applaud him. We know that the NBA will proudly support him, and that countless young LGBT Athletes now have a new hero."