Barbosa was signed for the veteran's minimum. The Celtics have 17 players on their roster, including two camp invitees.
A league source had confirmed the deal Wednesday. Yahoo! Sports earlier reported the agreement.
After cutting ties with first-year guards Dionte Christmas and Jamar Smith on Tuesday, the Celtics moved quickly to add Barbosa to the mix. The nine-year veteran gives their offseason haul another former Sixth Man of the Year (Jason Terry is the other) and allows Boston one more shoot-first guard who is also capable of handling the ball.
The Celtics initially expressed a desire to let a younger guard compete for a final backcourt spot, but when Christmas and Smith failed to distinguish themselves as potential ballhandlers this preseason, Boston altered its course and added Barbosa.
The 6-foot-3 Barbosa has averaged 12.5 points and 2.4 assists over 24.6 minutes per game in his career and averaged 8.9 points in 22 games last season with the Indiana Pacers. He'll join a backcourt that already includes Terry, Rajon Rondo, Courtney Lee, and Avery Bradley.
Celtics coach Doc Rivers already had suggested that Boston may have the deepest backcourt in the league when Bradley is healthy (he underwent double shoulder surgery and is projected to be a mid-December return).
Barbosa adds speed and scoring but can also handle the ball in Boston's by-committee approach. Rivers isn't worried about having a pure backup behind Rondo.
"Every team doesn't have two great point guards," Rivers said. "Most teams don't. I think when you count Courtney Lee and (Terry) and Avery, all three are ballhandlers -- none of the three are pure point guards. But they can certainly play the other."
Barbosa is the second Brazilian on Boston's roster, alongside rookie center Fab Melo.