Fitts, 24, was previously on the Utah Jazz where he averaged 0.9 points and 1.4 rebounds per game on a two-way deal. In his second NBA season, the 6-foot-8 big man has only played in 10 career games to date.
Martin, 26, has a little bit more experience than Fitts. The 6-foot-5 small forward has played in 27 games this season for the Indiana Pacers where he averaged a respectable 6.3 points per game, before being released in January.
These two signings leave the Celtics with one remaining roster spot on the 15-man roster after the team picked up Sam Hauser and Luke Kornet from the Maine Celtics.