Per club policy, terms of the deal were not disclosed.
"We are very excited to be able to add Mickael to our team," said Celtics president of basketball operations Danny Ainge. "His skills as a perimeter defender and an outside shooter provide great versatility to the roster."
The former first-round pick of the Golden State Warriors turned in averages of 7.2 points and 2.2 rebounds per contest in 57 games split between Orlando and Phoenix last season.
Pietrus is expected to fill a vacancy created because Jeff Green will be lost for the season following heart surgery.
But the 6-foot-6 Pietrus has his own injury issues to deal with, as a proposed deal involving the 29-year-old intended to ship him from Phoenix to Toronto two weeks ago was voided over concerns about his right knee, which underwent an arthroscopic procedure this past summer.
Over 496 NBA games since 2003, Pietrus has totaled 8.5 points and rebounds per game.