Markstrom, who was acquired from the Calgary Flames in a trade for defenseman Kevin Bahl and a first-round pick in 2025 on June 19, is 3-2-0 in his first five starts with the Devils.
Paul Cotter, Erik Haula and Nathan Bastian scored for New Jersey (5-2-0).
Brady Tkachuk scored, and Anton Forsberg made 33 saves for the Senators (2-2-0).
Haula gave the Devils a 1-0 lead at 2:37 of the first period when he tipped a Johnathan Kovacevic point shot past Forsberg.
Bastian made it 2-0 at 6:26 with a short-handed goal. He elected to shoot on a 2-on-1 and beat Forsberg with a wrist shot low blocker.
Cotter scored his fifth goal of the season to push it to 3-0 at 12:39 of the third period, dropping to one knee as he one-timed a rebound in the slot.
Tkachuk redirected Claude Giroux’s wrist shot from the slot past Markstrom on the power play with Ottawa’s net empty to cut it to 3-1 at 18:55.
