Tkachuk capitalized on a rebound off a Jordan Spence shot, stuffing the puck over the pads of Ilya Sorokin.
Shane Pinto scored short-handed, and Warren Foegele also scored for the Senators (35-24-9), who have won three of their past four games and are 12-3-2 in their past 17. James Reimer made 17 saves.
Matthew Schaefer scored to reach 50 points in his rookie season for the Islanders (39-24-5), who had won their two previous games. Brayden Schenn scored for the second straight game and Sorokin made 23 saves.
The Senators nearly opened the scoring at 17:50 of the first period, but a sprawling save by defenseman Carson Soucy in the crease denied Thomas Chabot on an open-net chance.
Schaefer gave New York a 1-0 lead at 4:45 of the second period on a point shot following a face-off win by Bo Horvat. The defenseman, the No. 1 pick in the 2025 NHL Draft, has 21 goals and 29 points.
Pinto tied it 1-1 with a short-handed goal at 10:27. He took a bank pass from Tyler Kleven at center ice to create a 2-on-1 with Claude Giroux, kept the puck, and beat Sorokin with a wrist shot.
Brayden Schenn restored the Islanders’ lead to 2-1 at 2:02 of the third period. Schenn carried the puck into the zone and dropped it back to Simon Holmstrom, who returned a no-look pass to Schenn at the right face-off dot. He beat Reimer with a shot from the circle.
Warren Foegele tied it 2-2 at 5:12. Fabian Zetterlund put a shot on net from the point that led to a scramble in front, where Foegele tipped the puck past Sorokin on the backhand.
