The Bruins (35-5-4) earned their third straight victory. The NHL’s only 30-win team improved to 7-1-0 in 2023.
Trailing 2-1, the Islanders went on their fourth power play of the game in the opening minute of the third period, but managed just on shot on goal during the advantage.
Minutes later, the Bruins went on their third power play, and Patrice Bergeron briefly left the game after he was hit in face by the puck on a slap shot from teammate David Pastrnak that deflected off the Islanders' Cal Clutterbuck at 4:26.
Bergeron then extended the Bruins' lead to 3-1 with a one-timer from the right faceoff dot off a pass from Pastrnak for his 15th at 5:03.
Parise hit a goalpost at 7:11, and Frederic sealed Boston's win when he tapped in a loose puck in front with 4:22 left for his 10th of the season.
Trailing 1-0 after 20 minutes, the Bruins took the lead with two goals 4:12 apart midway through the second.
McAvoy tied it with a one-timer from the top of the left circle off a pass from Matt Grzelcyk for his third at 7:48.
Boston then took the lead after Varlamov stopped Pavel Zacha's initial shot from the point. However, as the puck trickled toward the goal line, Forbort skated in from the left side and poked it in before Varlamov could knock it away. It was his third.
Parise scored with 4:19 left in the first on a one-timer from the top of the left circle off a pass from Sebastian Aho. It was his 13th of the season and first in five games.
Jean-Gabriel Pageau had an assist on Parise's first-period goal, giving him five assists and eight points in his last 10 games. ... The Islanders finished 0 for 6 on the power play and fell to 3 for 51 over their last 18 games.
The Islanders gave out bobbleheads of Bob Nystrom, the bruising forward who spent 14 seasons with the franchise and was a member of the teams that won four straight Stanley Cup championships from 1980-83. Nystrom scored the winning goal in overtime of Game 7 of the 1980 Finals to give the Islanders their first title.
William Dufour made his NHL debut for the Islanders after being called up from Bridgeport of the AHL. Dufour, a fifth-round draft pick in 2020 who turns 21 on Jan. 28, started at right wing on the top line with Josh Bailey and Mathew Barzal. He saw 6:48 of ice time, but did not play after he was on the ice for Boston's two goals in the second. ... D Dennis Cholowski made his Islanders debut. The 24-year-old had 10 goals and 20 assists in 115 NHL games over parts of four seasons with Detroit, Washington and Seattle.
Bruins: At the Rangers on Thursday night to complete a two-game trip to New York.
Islanders: At Buffalo on Thursday night
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Vin A. Cherwoo, The Associated Press