Axel Jonsson-Fjallby scored, and Vitek Vanecek made 21 saves for the Capitals (44-25-12), who also lost 4-1 to the Islanders on Tuesday and went 0-for-6 on the power play without Alex Ovechkin, who missed his second straight game because of an upper-body injury.
Washington, which will conclude its regular season against the New York Rangers on Friday, remained in the second wild card into the Stanley Cup Playoffs from the Eastern Conference, one point behind the Pittsburgh Penguins for third place in the Metropolitan.
"Special teams didn't go our way tonight," Capitals coach Peter Laviolette said. "It seemed like we had a lot of good looks, but we couldn't get to the interior. They are a tight team defensively and generated some chances 5-on-5. That was the difference in the game."
Josh Bailey had a goal and an assist, Mathew Barzal had three assists, and Semyon Varlamov made 26 saves for the Islanders (37-34-10), who went 3-for-5 on the power play.