Michael Cammalleri scored on a penalty shot 1:04 in. Maxim Lapierre and Brian Gionta also scored and Carey Price stopped 34 shots for Montreal, which remained in first place in the Northeast Division with 40 points.
Boston's Marc Savard had a goal and an assist, and Patrice Bergeron got his second assist of the game on Milan Lucic's goal with 4:50 left in the third. Blake Wheeler also scored and Tim Thomas made 37 saves for the Bruins, who now trail the Canadiens by four points.
Pacioretty, who made his season debut in Wednesday night's loss to Philadelphia, put Montreal up 3-1 with his first goal in over a year with 30 seconds left in the first. Gomez got his first of two assists when Pacioretty's backhand made its way in for his first goal since Nov. 25, 2009, ending a personal 28-game goal drought.
Pacioretty and Gomez both assisted on linemate Gionta's goal late in the second. The Canadiens' captain got his 11th at 16:54 to give Montreal its third two-goal lead at 4-2.
Savard scored his first goal in seven games in the second and assisted on Bergeron's power-play goal 15:10 into the third to draw Boston within one for the third time in the game.
The Canadiens had not lost more than two games in a row prior to their losing streak.
Gomez had two assists for a second night in a row after missing Montreal's two previous games because of a lower-body injury. He had nine points in 28 games before his injury.
The 21,273 on hand at the Bell Centre cheered their approval when referee Bill McCreary pointed to the center-ice faceoff circle after Cammalleri was hooked by Bruins captain Zdeno Chara on a breakaway just over a minute in.
Thomas moved aggressively around the goalmouth as Cammalleri skated in on the penalty shot. The Canadiens' left wing deked and fired a shot over Thomas' outstretched right pad for his 11th goal, touching off an even larger roar from the sellout crowd.
Lapierre made it 2-0 with his fifth at 6:24 on a shot off the right post that went in off Thomas.
Wheeler cut the lead to 2-1 at 15:52 with a shot from the right edge of the goalmouth that trickled down Price's pad and just made it across the goal line.
Canadiens defenseman P.K. Subban brought the crowd to life again later in the period. The Montreal rookie backed into a solid open-ice hit on Brad Marchand, drawing the ire of several of the Bruins, including Gregory Campbell, who drew a roughing minor, the only penalty on the play.
Savard drew Boston within one once again 6:54 into the second when he deflected Andrew Ference's shot past Price to make it 3-2.
Notes
An unlikely fight pairing midway through the second saw Bruins center David Krejci and Cammalleri drop the gloves and go at it. Kreji had three minor penalties in his 22 previous games this season. … D Yannick Weber and C Tom Pyatt did not dress for Montreal.