Tuukka Rask stopped 35 shots to help the Bruins win for the third time in four games (3-0-1) following a season-high four-game skid.
Rasmus Ristolainen and Brandon Montour each scored and Linus Ullmark had 24 saves for the Sabres. Buffalo has dropped nine of 10 (1-7-2) after winning nine of its first 12 games.
Bruins star centre Patrice Bergeron had an assist in his return after missing two games with a lower body injury.
Marchand’s second of the game and 15th of the season came on a power play with 5:15 left in the second period to put Boston up 2-1. Matt Grzelcyk skated into the slot and fired a wrist shot that bounced right to Marchand, who threw the puck in under Ullmark.
Pastrnak’s second-chance, power-play strike made it 3-1 two minutes into the third.
Rask wowed the crowd with a diving glove save on Evan Rodrigues’ shot at 4:51.
Montour’s one-timer from the top of the left circle brought the Sabres to 3-2 with 7:02 remaining.
Buffalo led 1-0 after Ristolainen scored his first of the season on a rebound of Jack Eichel’s shot from the left circle 5:25 into the game.
The Sabres pelted Rask with 13 shots to open the game before Brandon Carlo logged the Bruins’ first shot with 7:49 left in the first.
Boston tied it on its second shot after Marchand tipped in Zdeno Chara’s shot from the blue line with 6:08 to play in the first.
Buffalo finished the first with a 17-4 advantage on shots.
NOTES: Marchand and Pastrnak (19 goals) are the first teammates with 15 goals apiece in their team’s first 22 games since Tampa Bay’s Steven Stamkos and Ryan Malone in 2009-10, according to Sportsnet Stats. … Boston remained the only NHL team without a regulation home loss this season at 8-0-4. … Bruins C Par Lindholm exited after being taken down by Rasmus Asplund while fighting for a loose puck early in the first period, but returned in the second. … Boston RW Brett Ritchie sat out with a recurrence of an infection. … Boston D Torey Krug (upper body) missed his fifth straight game, but participated in an optional skate Thursday and hopes to return Saturday. … Sabres LW Johan Larsson was back in the lineup after a three-game absence with an upper-body injury. … Bruins trainer Don DelNegro worked his 2,000th NHL game in his 27th season. DelNegro was honoured with a video tribute and an ovation midway through the first period.
Sabres: At Florida on Sunday night.
Bruins: Host Minnesota on Saturday night.