T.J. Oshie and Nicklas Backstrom each had two goals and an assist for the Capitals, who have won four games in a row. Braden Holtby made 25 saves.
"They've got a good team, they are the best team in the League right now," Washington forward Tom Wilson said. "They have had our number this year so far (Tampa Bay won the first two games), so we wanted to come in tonight and just feel good at our game, and I think we did that tonight."
J.T. Miller, Tyler Johnson and Nikita Kucherov scored for the Lightning (59-15-4), who have four games remaining to try to become the third team in NHL history to win 60 games (Detroit Red Wings, 62-13 with seven ties in 1995-96; Montreal Canadiens 60-8 with 12 ties in 1976-77). Andrei Vasilevskiy made 29 saves.
"We just didn't play well enough to win," Tampa Bay captain Steven Stamkos said. "Whether we're playing Washington, [the] Boston [Bruins], [the] Toronto [Maple Leafs], whatever, you play the way we did the first half of the game, you're not going to win."
Lightning defenseman Victor Hedman did not play after the first period because of an upper-body injury, and defenseman Erik Cernak did not return after a fight with Wilson at 12:13 of the third.
"Hedman got hit in a, kind of, tough fashion, so we'll re-evaluate him," coach Jon Cooper said. "But [Cernak] is going to be OK."
Backstrom gave the Capitals a 1-0 lead at 7:24 of the first period with a one-timer from the slot off a pass from Wilson.