Nicklas Backstrom and Garnet Hathaway scored, and Darcy Kuemper made 27 saves for Washington (28-20-6), which has won three of four.
"It was a good game," Capitals coach Peter Laviolette said. "I thought we played a smart [first] period. I thought we defended well. Couple breakdowns, but for the most part defended pretty, pretty well. [We were] able to get a lead, we get a power-play goal, so there were some positives to come out of the first period."
Nick Foligno scored, and Jeremy Swayman made 21 saves for Boston (39-8-5), which lost for the second time in regulation at home (22-2-3) this season.
"I thought our effort was good," Bruins coach Jim Montgomery said after Boston's first game since Feb. 1. "You expect it to be a little sloppy; it was sloppy in the first. We got better, as we expected to get better. We were really good in the second. I didn't love the beginning of our third, but still, the effort's there. The execution's going to come."
The Capitals last played Jan. 31.
"I don't think we had a bad game," Bruins forward Brad Marchand said. "We missed a couple opportunities, and if we capitalized, I think it's a different game."
Backstrom gave the Capitals a 1-0 lead at 6:28 of the first period with a 5-on-3 power-play goal. Marcus Johansson's shot on the doorstep bounced off Swayman to Backstrom, who scored from the right side.
"It's always important to get the first goal," Backstrom said. "You need to score those goals [in] those situations, so it was a good start for us."
Hathaway made it 2-0 at 11:40 of the second period, getting to a loose puck at the blue line and shooting over Swayman's shoulder from the high slot.
"They're a good team in the neutral zone," Hathaway said. "They defend quickly, so we're trying to get a battle on the boards and get into the zone, and then attack their [defensemen]. … The puck squeaks out to me, and I'm in the slot, so like a lot of us are thinking, 'Let's shoot it.'"