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.