Evgeni Nabokov recorded his first shutout of 0.
John Tavares netted a goal to extend his point streak to 10 consecutive games for the Islanders, who had lost four of six coming in.
Nabokov made 17 saves for his 51st career shutout.
Tomas Vokoun stopped 25 shots for the Capitals, who finished 3-1 on their four-game homestand. Washington will play six of its next seven on the road.
The Islanders struck first on the power play with 7:55 left in the first period. From the high slot, Marc Streit made a slick slap-pass to Tavares lurking right of the net and he tipped it home.
Parenteau ripped a wrister past Vokoun's glove side for a 2-0 Islanders advantage at the 6:05 mark of the second period.
The Islanders added another power-play goal with 7:22 remaining in the final period. Matt Moulson found Parenteau in the slot from behind the net, and Parenteau snapped it into the back of the net to make it 3-0.
The Capitals finished 0-for-3 on the man advantage and had just 10 shots on goal heading into the third period.
Tavares has six goals and 11 assists during his 10-game point streak. The No. 1 overall pick of the 2009 draft is also leading New York with 44 points this year...The Islanders improved to 7-10-3 on the road this season...Washington dropped to 17-6-1 as the host.