The New York Islanders needed just 15 seconds of overtime to net the game-winner.
John Tavares put home the rebound off of a Nikolay Kulemin shot for his first of the playoffs.
The Islanders’ captain had a league-high four overtime winners in the regular season.
New York dominated the first two periods out-shooting the Caps 33-13.
Kyle Okposo’s second of the playoffs gave the Islanders a 1-0 lead after two periods.
Nicklas Backstrom tied it 1-1 at 13:54 of the third to force overtime.
Capitals forward Eric Fehr left the game early in the first period with an upper body injury following a hit from Okposo and did not return.
Braden Holtby, who missed Game 2 due to illness, made 40 saves in the loss.
Jaroslav Halak stopped 24 shots for the win.