Pierce made up for the absence of injured All Star point guard Rajon Rondo by dishing out 14 assists, but it was his pull-up jumper with seven seconds left that gave the Celtics a four-point lead and some breathing space.
Pierce added two free throws for the game's final points, to finish with a game-high 29, as Boston (37-26) wrapped up the division and secured at least fourth in the Eastern Conference.
Orlando (36-26), sixth in the East, trailed by 13 with about seven minutes to go but rallied to within 98-96 on a Glen Davis breakaway dunk with 31 seconds left.
Davis finished with 27 points and Jameer Nelson added 21 for Orlando, who were without the struggling Dwight Howard as he sat out the game with an ailing back.
(Writing by Jahmal Corner in Los Angeles; Editing by Peter Rutherford)