Mitchell Chaffee, Gage Goncalves and Brandon Hagel scored for the Lightning (35-20-4), and Andrei Vasilevskiy made 20 saves.
“That's the way we've got to play,” Tampa Bay captain Victor Hedman said. “Defense wins championships. It's a long way from that, but obviously playing this way, you've got to feel comfortable playing in a 1-0 game. It's fun.”
Logan Thompson made 16 saves for the Capitals (38-14-8), who have lost three in a row for the first time this season.
“It’s three games,” Washington forward Tom Wilson said. “We don’t like it, but we’ve had a consistent year, so there’s going to be ups and downs.
“We’re not panicking right now. We love the group in here and we’ve just got to find a way to get back on track.”
The Capitals appeared to take a 1-0 lead at 5:40 of the first period when Jakob Chychrun scored from the point, but the Lightning successfully challenged that Chychrun played the puck with a high stick earlier in the sequence.
Chafee gave Tampa Bay a 1-0 lead on a power play at 17:11 when he scored from the bottom of the left circle on a rebound of Brayden Point’s shot.
“Obviously, they're a tremendous team,” Chafee said. “We come into this game, and we knew it's going to be tight. It's going to be a hard-checking game, and that's what it was. And it's capitalizing on your chances.”