Rookie Braden Holtby stopped 31 shots for Washington in Game 7 – the seventh one-goal game of the series. Matt Hendricks scored the Capitals’ other goal.
Tyler Seguin scored, and Tim Thomas made 26 saves for Boston.
It was the first time in NHL history that a playoff series had seven games determined by one goal. Four of the games went to overtime.
Hendricks scored midway through the first period, and Seguin tied it in the second. It stayed that way through a scoreless third, with Washington killing off a penalty in the final 3 minutes to send the game into overtime.
The Capitals broke into the Boston zone with former Bruin Mike Knuble leading a 2-on-1.
Knuble shot, and Thomas left the rebound out where Ward could reach it with his backhander.
The building fell silent as the Capitals celebrated just their third postseason series win since a run to the Stanley Cup finals in 1998. Some fans littered the ice with debris, but the Bruins waited for the postgame handshake.
Thomas, bringing up the rear, gave Holtby a tap on the shoulder and said, “Great job, kid.”
No team has repeated as Stanley Cup champions since the Detroit Red Wings in 1997-98.