It was Pastrnak's sixth NHL hat trick, second this season (four goals in 4-2 win against Anaheim Ducks on Oct. 14).
Jaroslav Halak made 36 saves, and Brad Marchand and Charlie Coyle each had a goal and two assists for Boston (16-3-5), which has an eight-game point streak (5-0-3).
[WATCH: All Bruins vs. Canadiens highlights]
The Bruins, who complete a two-game road trip at the Ottawa Senators on Wednesday, had 12 players with at least one point and six who had a multipoint game.
"It's good that a lot of guys got on the score sheet," said Pastrnak, who leads the NHL with 23 goals. "Obviously, two points is two points, no matter how you get there. You know, if you 1-0 or 8-1, it doesn't matter for tomorrow. But you can enjoy it tonight and get some confidence and wash it off tomorrow, and just like same if we lose and be ready for tomorrow night."
Video: Enterprise Hat Trick: Pastrnak nets 3 vs. Canadiens
Shea Weber scored for Montreal (11-8-5), whose season-high winless streak reached five games (0-3-2). The Canadiens have allowed 14 goals in their past two games, including a 6-5 loss to the New York Rangers on Saturday, when they blew a 4-0 lead.
"We've got to hit the reset button and fight our way out of this because no team's going to make it easy for us," defenseman Jeff Petry said. "Everyone's going to have that mindset, when they're down, to step on them. And it's our responsibility to try to figure it out within these walls and find a way to battle it out together."
Pastrnak scored one of three Boston goals in the first period and gave them a 4-1 lead with his second of the game eight seconds into the second.
Canadiens goalie Carey Price was pulled after Anders Bjork made it 5-1 at 1:10.
Pastrnak put the Bruins up 6-1 when he beat goalie Keith Kinkaid at 9:06 to complete the first regular-season hat trick by a Boston player in Montreal since Steve Kasper scored three times in a 6-4 loss Nov. 30, 1987.
Sean Kuraly got his third assist of the game when Coyle made it 7-1 at 8:26 of the third. Jack Studnicka got an assist in his first NHL game on Danton Heinen's goal that put the Bruins up 8-1 at 17:58.
Video: BOS@MTL: Coyle hammers home Krug's stellar pass
"I know it's a meaningless goal at the end, but for [Studnicka] it's very meaningful, he gets an assist on it," Boston coach Bruce Cassidy said. "You're an offensive guy, you want to get your first NHL point, so that's out of the way. So good for him, in that regard."
Price, who made six saves on 11 shots, was pulled for the first time in 113 starts dating to Dec. 9, 2017, when he allowed the first four goals in a 6-2 loss to the Edmonton Oilers. Kinkaid made 10 saves.
DeBrusk gave Boston a 1-0 lead with a power-play goal at 8:03. Weber tied it 1-1 at 12:41.
Pastrnak made it 2-1 when he one-timed a Coyle pass for Boston's second straight power-play goal at 14:24.
Marchand made it 3-1 when he scored with 37 seconds remaining.
"Obviously our power play got us some goals, and it's been good for us all year in that regard, but Halak made some big saves," Cassidy said. "And we weren't sharp, we weren't as hard on the puck as we need to be. They were. And then we were able to finish some nice plays. And then the goal at the end of the period probably hurt them. That's a goal that you hate to give up, no matter in any circumstance, so to go down two, I'm sure they felt they played well 5-on-5, so that was probably a tough one for them."
Video: BOS@MTL: Marchand pots backhander off of turnover
They said it
"To be honest, I felt absolutely (horrible); the first couple of shifts I felt really bad. Sometimes, you feel good, and it doesn't (go in). But that's just hockey. It's a big up and down game, and I was just trying to play my game." -- Bruins forward David Pastrnak
"I think with us right now it's not the amount of mistakes, it's the type of mistakes we're making. I think we're making mistakes where they're getting opportunities that are very, very high-danger chances. And you know, we're really leaving our goaltending out to dry right now. It's just about being more alert, sharper, and you just can't make the mistakes that we're making right now. And it's on us to learn from it and get better." -- Canadiens forward Brendan Gallagher
Need to know
Pastrnak's 23 goals are the most by an NHL player through 24 games of a season since Mario Lemieux had 26 for the Pittsburgh Penguins in 1992-93. ... It was the seventh time Boston scored eight goals in Montreal, first since Oct. 28, 1998. ... Pastrnak's goal eight seconds into the second period was two seconds shy of the fastest to begin a period in Bruins history (three times). ... Marchand is the fourth Boston player in 45 years to reach 40 points in 24 or fewer games to start a season (Bobby Orr, 21 games, and Phil Esposito, 22, in 1974-75; Adam Oates, 24, in 1992-93). He has 42 (17 goals, 25 assists) in 24, third in the NHL behind Edmonton Oilers linemates Leon Draisaitl (48) and Connor McDavid (47).
Bruins: At the Ottawa Senators on Wednesday (7 p.m. ET; SN360, TVAS, NESN, NHL.TV)
Canadiens: Host the New Jersey Devils on Thursday (7:30 p.m. ET; NHLN, SN, RDS, MSG+, NHL.TV)
Video: Pastrnak lifts Bruins over Canadiens with hat trick