Pierce hit a pair of crucial 3-pointers down the stretch, including a 25-footer with 52 seconds remaining, to help the Celtics overcome a 13-point deficit and rally for a 3-2 lead over the Heat in the Eastern Conference Finals.
Kevin Garnett netted a team-high 26 points and grabbed 11 rebounds while Rajon Rondo dished off 13 assists to go with seven points and six boards for the Celtics, who moved to within one win of returning to the NBA Finals.
Boston notched the first road victory in the series with the help of 13 points each from Ray Allen and Mickael Pietrus.
Miami was led by LeBron James with 30 points and 13 rebounds while Dwyane Wade added 27 points in the loss.
Udonis Haslem grabbed 14 rebounds for the Heat, who will try to avoid elimination in Thursday's Game 6 at Boston.