Tomas Plekanec scored in the third period to put the Habs ahead 2-0, and Pacioretty followed just 80 seconds later with his second goal of the game.
Carey Price helped Montreal improve to 5-1-1 in its last seven games with 29 saves.
James Neal scored twice for Pittsburgh, which had its own three-game winning streak broken.
Marc-Andre Fleury made 26 saves.
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