DeBrusk scored to tie the game 1-1 with 12:14 remaining and scored the winner with 2:24 left. They were his first two goals in an NHL outdoor game. He has played in three.
The Penguins thought Evgeni Malkin might have scored a last second tying goal, but it was immediately waved off because time had expired.
"You want to win," Bruins coach Jim Montgomery said. "It's filled with Bruins fans. It's an incredible atmosphere. We're at Fenway Park. So, for that reason it's great to come out with the W. And, again, in the third period this team finds ways to raise their level and take over a game."
Linus Ullmark, playing in his first NHL outdoor game, made 26 saves for the Bruins (29-4-4), who extended their home point streak to 22 games (19-0-3), the longest in team history to start a season, breaking the 21-game record set in 1973-74.
Boston is 8-0-3 in its past 11 games.
"It's just pure joy and happiness," Ullmark said. "It's something I've dreamt about for [15] years, since I saw the first one [2008 NHL Winter Classic] at home in Sweden. I always dreamt about it and I was always very jealous of the people that played before me and had an opportunity. Now we had the opportunity. Like Jake said, it's a dream come true."