Anders Lee, Anthony Beauvillier and Brock Nelson also scored goals for the Islanders, who snapped a three-game losing streak.
Jaroslav Halak stopped 33 shots to pick up his first win in 11 starts. It was just the third win in 17 games (3-10-4) for New York, which was officially eliminated from playoff contention on Monday night with a 3-0 loss to the Florida Panthers.
Matt Duchene and Bobby Ryan each had a goal and two assists and Erik Karlsson tallied his ninth goal of the season for Ottawa. Mike Condon finished with 27 saves while falling to 5-16-5 on the season.
Lee opened the scoring with his team-leading 39th goal of the season, backhanding in a rebound of a John Tavares shot from in front of the crease.
Ottawa rebounded to a take a 2-1 lead after one period on goals by Duchene, who deflected a one-time shot by Jean-Gabriel Pageau for his 24th goal, and by Karlsson, who fired a wrist shot past Halak's glove from the right circle off a nifty between-the-legs drop pass by Duchene.
The Islanders, despite being outshot 13-7, regained the lead with a pair of second-period goals.
Beauvillier tied it with his 17th off a nice pass from Calder Trophy favorite Mathew Barzal, who finished with two assists to give him 58 helpers and 78 points on the season, the most points for any rookie in the NHL.
Nelson then scored his 18th with a wrist shot from the top of the left circle to give New York a 3-2 lead with less than two minutes remaining in the second period.
Ottawa tied it with 4:34 left when Islander defenseman Brandon Davidson deflected Ryan's pass through Halak's pads.
Ladd answered with the game-winner, redirecting Thomas Hickey's shot from the left point past Condon.
--Field Level Media
NY Islanders | Ottawa | |
Andrew Ladd 2 | Points | Matt Duchene 3 |
Andrew Ladd 1 | Goals | Matt Duchene 1 |
Mathew Barzal 2 | Assists | Matt Duchene 2 |
Anders Lee 1 | Power Play Goals | Matt Duchene 1 |
N/A | Short Handed Goals | N/A |
Jaroslav Halak .917 | Save Percentage | Mike Condon .871 |
Jaroslav Halak 33 | Saves | Mike Condon 27 |
Team | Shots | Goals | Power Play | Penalty Kill | Penalty Mins | Face Offs Won |
NY Islanders | 31 | 4 | 1-2 | 3-4 | 13 | 31 |
Ottawa | 36 | 3 | 1-4 | 1-2 | 9 | 26 |