T.J. Oshie, Jay Beagle and Evgeny Kuznetsov scored in a 7:10 span of the first period, and John Carlson added a goal for the Capitals. Kuznetsov and Carlson also each had three assists, and Oshie had two.
Braden Holtby made 22 saves for Washington.
The Islanders will open their first season in Brooklyn on Friday night at home against Chicago. The Capitals will start the season at home Saturday night against New Jersey.