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.
White House spokesman Jay Carney called the decision courageous and former President Bill Clinton said it was "an important moment for professional sports and in the history of the LGBT community."
"I hope that everyone, particularly Jason's colleagues in the NBA, the media and his many fans extend to him their support and the respect he has earned," Clinton added.
Daughter Chelsea, who knew the player from Stanford, 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 NBA players voiced support, including Kobe Bryant, who tweeted that he was proud of Collins.
"Don't suffocate who u r because of the ignorance of others," his post said, followed by the words "courage" and "support."
Wizards President Ernie Grunfeld weighed in saying: "We are extremely proud of Jason and support his decision to live his life proudly and openly. He has been a leader on and off the court and an outstanding teammate throughout his NBA career. Those qualities will continue to serve him both as a player and as a positive role model for others of all sexual orientation."
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.
Collins wrote that he quietly made a statement for gay rights even while keeping his sexual orientation a secret. He wore the No. 98 with the Celtics and Wizards — that was the year Matthew Shepard, a gay college student in Wyoming, was killed, and the Trevor Project, a suicide prevention organization, was founded.
" 'Courage' and 'inspiration' are words that get thrown around a lot in sports, but Jason Collins has given both ideas a brand new context," said Aaron McQuade, who heads the sports program for the advocacy group GLAAD, "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."
In February, former U.S. soccer national team player Robbie Rogers said he was gay — and retired at the same time. Rogers is just 25, and others have urged him to resume his career.
"I feel a movement coming," he tweeted after the Collins news broke.
Female athletes have found more acceptance in coming out; Brittney Griner, one of the best women's basketball players, caused little ripple when she acknowledged earlier this month she was a lesbian. Tennis great Martina Navratilova tweeted Monday that Collins is "a brave man."
"1981 was the year for me- 2013 is the year for you," her post added.