BOSTON — Brady Tkachuk scored 21 seconds into overtime for the Ottawa Senators in a 3-2 win against the Boston Bruins at TD Garden on Saturday.

Bruins forward Elias Lindholm had a shot stopped by Senators goalie Linus Ullmark, which led to a 2-on-1 rush the other way for Ottawa, which Tkachuk finished off with his 12th shot on goal of the game.

“I think the last two games for myself, individually, I haven’t liked the way I’ve been playing with the puck,” Tkachuk said. “It’s just something that I want to focus on, that every time I had an opportunity to shoot – that I’m pretty confident in my shot – that every time I have the puck on my stick, that has a chance to go in, so I have to start utilizing that more.”

Tkachuk also had an assist, and Josh Norris and Michael Amadio scored. Ullmark made 14 saves for Ottawa (7-7-0), which has won three of its past six (3-3-0).

“We’ve challenged our guys the last couple days here to play more consistent,” Senators coach Travis Green said. “Play faster, play harder, and they responded in a really good game for 60 minutes– 61 [with overtime.]”

It was Ullmark’s first time facing Boston after spending three seasons there from 2021-24. The Bruins traded Ullmark to Ottawa on June 24 in exchange for goalie Joonas Korpisalo, forward Mark Kastelic and a first-round pick in the 2024 NHL Draft, which they used to select Dean Letourneau (No. 25).

“There was certainly a lot [of emotions], not going to lie, there was a lot,” Ullmark said. “Especially during warmups. I had goosebumps going out there, hearing the crowd again, and also just getting the win in this barn is not easy. It’s something that you dream of, especially when you’re coming in here as an opponent as well, and you really want to bring your A-game. You want to do that, and we haven’t really done that lately, but I thought that today we really showed what kind of a team we are.”

Said Bruins goalie Jeremy Swayman, who spent the last three years sharing the crease with Ullmark in Boston: “We share such an incredible bond, and seeing him in another jersey’s different. I’m very happy for him and the success he’s having, and I expected a game like that back and forth, so it’s fun to watch him succeed.”

Pavel Zacha and Brad Marchand scored, and Swayman made 31 saves for Boston (7-7-2), which has lost two of its past three (1-1-1).

“That’s how it goes,” Marchand said. “Nobody can write the perfect script, how things are going to play out. We’re just going to take it day by day, and obviously we want to get better. We want to try to improve everyday, but it’s just not happening right now.”

Norris gave the Senators a 1-0 lead with 7.8 seconds left in the first period, sneaking a one-timer under the crossbar on the power play.

Zacha tied it 1-1 at 4:51 of the second period with a spinning backhand shot after Nikita Zadorov kept the puck alive in the offensive zone.

“You see that they have way more [offensive] zone time,” Zacha said. “In games like that, we didn’t deserve to win, even in overtime.”

Marchand gave the Bruins a 2-1 lead 15 seconds later, elevating a cross-crease feed from Justin Brazeau.

Amadio tied it 2-2 at 17:16 of the second with his first goal of the season, skating in and snapping a shot past Swayman from behind the right circle.

“It took a few games longer than I would have liked, but it’s nice to get the monkey off the back there,” Amadio said. “Now I can start playing a little more freely out there.”

Ottawa outshot Boston 12-0 in the third period.

“[We’ll] talk about it, figure out why,” Bruins coach Jim Montgomery said. “We’ll look at what we’re doing sports science-wise, but we’ll come up with answers. Right now, I don’t have an answer for you.”

NOTES: Tkachuk tallied his eighth career overtime goal to pass Mike Fisher (7) for the most in franchise history. … Ullmark and Swayman squared off once before Saturday, in a 3-2 shootout win for Boston against the Buffalo Sabres on April 13, 2021. Ullmark logged 4:54 for Buffalo in the first period before exiting the game due to injury. … Marchand scored his 290th even-strength goal, passing Phil Esposito for second-most in Bruins history behind Johnny Bucyk (384).