BOSTON — Matt Duchene and Tyler Seguin each had a goal and two assists for the Dallas Stars in a 5-2 win against the Boston Bruins at TD Garden on Thursday.

Mason Marchment had two assists, and Casey DeSmith made 25 saves for Dallas (6-2-0). Jason Robertson scored in his 300th NHL game.

“There was a quiet confidence in the group,” Stars coach Pete DeBoer said. “We had no passengers tonight. I thought everyone was engaged and involved, and a real good team — probably our best team game of the year.”

The Stars scored three power-play goals in the second period, which tied a team record for most in a period since the franchise’s move to Dallas in 1993.

“It’s exciting to see some power-play goals go in,” Seguin said. “We’ve been working hard at it, probably gripping our sticks too much at it. … We’ve just been working through it, and it’s nice to see some results for the players.”

David Pastrnak and Justin Brazeau scored, and Jeremy Swayman made 27 saves for Boston (3-4-1), which has lost three straight (0-2-1).

“If you’re not focused on habits and details, you’re going to get frustrated,” Bruins coach Jim Montgomery said. “The margins of error aren’t very big in this league. And we’re on the wrong side right now, and I think our attitudes are driving it.”

Pastrnak gave the Bruins a 1-0 lead at 11:57 of the first period during 4-on-4 play. Matthew Poitras intercepted Stars defenseman Mathew Dumba’s pass behind the net before teeing up Pastrnak for a one-timer in the slot.

“We came out slow, and we can’t wait until the second half of the game to find our legs and find a way to push,” Poitras said.

Duchene tied it 1-1 at 16:20 of the first, taking Seguin’s cross-ice pass wide and elevating the puck past Swayman from the left face-off circle.

“We wanted to get back to Dallas Stars hockey,” Duchene said. “Every single guy was going tonight. Even when we got down one [goal], we really liked our 5-on-5 game in the first [period], and we just want to keep going.”

Robertson gave the Stars a 2-1 lead 31 seconds into the second period, finishing off Jamie Benn’s behind-the-net feed on the power play.

Logan Stankoven made it 3-1 at 4:57 with another power-play goal. Duchene skated in unimpeded from the left side and found him in front.

“We’ve got to stop taking penalties, and that’s myself included,” Bruins defenseman Charlie McAvoy said. “… Most of the time, you can always draw it back to effort. We’re taxing our penalty kill way too much.”

Seguin pushed it to 4-1 with another power-play goal at 8:59 after he was left open with plenty of space on the right side to beat Swayman with a wrist shot.

Brazeau cut it to 4-2 at 10:41 on the power play, finishing off Trent Frederic’s wraparound attempt.

“When everybody’s playing the right way, usually everyone enjoys success and benefits from it,” Bruins captain Brad Marchand said. “We’re kind of right around it, and it’s going to come. But us as leaders and older guys, we need to be better and we need to set the tempo for that and kind of lead by example.”

The Bruins outshot the Stars 10-7 in the third period, but Roope Hintz scored an empty-net goal for Dallas at 17:58 for the 5-2 final.

“When you’re trying to hold on to a lead, it’s tough because the other team brings everything they have,” DeSmith said. “Not that we sat back, but playing a little bit more careful and calculated. So I just thought they were a little bit desperate and they brought it in [the third] period.”

NOTES: DeBoer won his 618th game in the NHL, moving into sole possession of 21st place on the all-time list and breaking a tie with Bruce Boudreau and Jacques Lemaire. … Robertson scored his 137th goal, the second-most by a Stars/Minnesota North Stars player through 300 games, behind Dino Ciccarelli (158). … It was also Hintz’s 400th NHL game. … Dallas had been 2-for-21 on the power play through its first seven games before going 3-for-5 on Thursday. … The Stars had also gone six games without allowing a power-play goal — leading the League going into the game at 95.5 percent on the penalty kill — before allowing Brazeau’s goal. … Pastrnak scored his 78th game-opening goal, the most among all players dating to his debut season in 2014-15. … Marchand has not scored a goal through eight games this season. It’s his longest season-opening goal drought since 2010-11, when it took nine games to score.