Mike Green supplied two goals and an assist as Southeast Division champion Washington ended the lockout-shortened season with a 27-18-3 mark and entered the playoffs with the third seed in the Eastern Conference.
Braden Holtby turned aside 32 shots for the Caps.
Tuukka Rask stopped 34-of-37 tries for Boston, which trails Northwest Division-leading Montreal by one point heading into Sunday's season finale against the Ottawa.
The game was rescheduled from April 15 due to the Boston Marathon bombings.
Milan Lucic provided a goal and an assist in the loss.