Jason Collins -- GAY KICKBALLIN' ... During NBA Training Camp

With NBA training camp officially underway, veteran free agent Jason Collins has finally joined a team -- in a gay kickball league ... TMZ has learned.

The 34-year-old took the field in the Varsity Gay kickball league in Hollywood this week -- playing 1st base for "Team Twerk."

Collins was a little rusty on the field -- dropping an easy pop fly in the beginning of the game, but our gay kickball sources tell us he quickly got his act together and played like a stud the rest of the game.

In the end, Team Twerk emerged victorious -- pulling out an 11 to 5 victory.

We're told Jason isn't fully committed to the team, since he's still trying to sign with an NBA squad -- but he's the go-to substitute when the team is short on players.