Milan Lucic added two goals and an assist for Boston, while David Krejci had a goal and two assists. Shawn Thornton also scored and Patrice Bergeron added three assists for the defending Stanley Cup champions, who seem be finding their game after a sluggish start to the season.
With No. 1 goalie James Reimer still out with concussion-like symptoms, Ben Scrivens was unable to recapture the magic of his NHL debut Thursday when he stopped 38-of-39 shots in Toronto's 4-1 win at Columbus. The Leafs rookie allowed five goals on 14 shots against the Bruins, who won two in a row for the first time this season.
Scrivens was replaced by Jonas Gustavsson after the fifth Boston goal. He finished with four saves and was jeered by the crowd in the third period after the seventh Boston goal.
The Bruins (5-7-0) took control of the game early in the second when Seguin and Lucic scored eight seconds apart.
Seguin gave the Bruins a 2-0 lead just 34 seconds into the middle period with a great display of hand-eye co-ordination. The 19-year-old batted a deflected Bergeron pass off the rush out of midair past a helpless Scrivens.
Lucic stretched the Boston lead to three moments later off a great feed from Zdeno Chara. The Bruins captain found Lucic with a tap-to-tap pass up the middle off the faceoff at centre and the hulking winger split the Toronto defence before wiring a shot over Scrivens' shoulder.
Toronto coach Ron Wilson used his timeout after the goal, but it did little to stem the Boston tide as the Bruins continued to carry the play.
Seguin, who took a big hit from Joey Crabb earlier in the period, completed his hat trick at 15:01 when he beat Scrivens with a snapshot shortside and Krejci made it 5-0 nothing just 14 seconds off a feed from Lucic.
That was the end of a disappointing night for Scrivens, who flew in family and friends from Spruce Grove, Alta., to see his first home game with Toronto (9-4-1).
Lucic made it 6-0 off a pass from Krejci at 5:44 of the third and Thornton banged home his first of the season for Boston's seventh 28 seconds later as boos rained down from the Air Canada Centre crowd.
Seguin, who the Bruins drafted second overall with one of the picks acquired in the Phil Kessel trade, showed Maple Leafs fans his talent on a big stage. Kessel, meanwhile, continued to struggle against his former team, registering just two shots.
Kessel has 10 goals and 11 assists on the season. He's been held pointless in just three times this season, but two of those games are against Boston.
Unfortunately for Toronto, Kessel was far from the only culprit on a night when very few Maple Leafs showed much willingness to get to the front of the net or pay the price.
The Bruins started the season 3-7-0 but beat Ottawa on Tuesday after back-to-back losses to Montreal and defeated another Northeast Division opponent in Toronto to get out of the Eastern Conference basement.
Boston took the lead on the power play at 6:23 of the first. With Crabb off in the box for holding, the Bruins worked it around nicely to Seguin, who wired a one-timer past Scrivens from the right faceoff dot.
A sluggish opening period was a sign of things to come for the Leafs, who only got their legs moving as the period wore on. Jay Rosehill had a puck skip over his stick in front and Nikolai Kulemin couldn't find the range off a scramble with Thomas out of position.
Boston finished the night 1-for-2 with the power play, while Toronto wound up 0-for-3.
Notes: Defenceman Luke Schenn, centre Philippe Dupuis and right-winger Colton Orr were scratched by the Maple Leafs. ... Defenceman Steven Kampfer and left-winger Benoit Pouliot sat out for Boston. ... The Leafs wore their heritage jersey. ... Attendance was 19,497. ... The Bruins spent Friday at the Hockey Hall of Fame signing autographs for fans. The team will also donate a Stanley Cup ring to the Hall. ... Toronto centre Tim Connolly needs one more point for 400 in his career.