Rask and the Bruins fell behind 1-0 in the second period, but he shut the Flames out in the third period while the Boston offense scored two goals on 15 shots.
David Krejci tied the contest on a power-play goal at 13:49 and Reilly Smith won it 98 seconds later with a wrist-shot goal at 15:27. The win was Boston's third in a row.
Jiri Hudler scored the lone goal for the Flames. Reto Berra turned in 22 saves in goal.
Tuesday marked the return to Calgary of longtime Flames captain Jarome Iginla, who was traded to Pittsburgh last spring and joined the Bruins this season. Iginla was given a warm welcome by the fans and took four shots in the game.