The veteran spent last season with the Minnesota Timberwolves and Atlanta Hawks. He averaged 10.9 points and 5.2 assists in 24.8 minutes per game and shot 36.8% from beyond the arc.
The 32-year-old provides depth at point guard for the Celtics. He's set to join Tristan Thompson as another recent veteran addition to the team.
Teague was an NBA All-Star in 2015.