The Boston defenseman had taken 44 shots on goal without a score this year before blasting a slap shot past Robin Lehner.
That broke up a goaltending duel between Lehner and Boston's Anton Khudobin, who made 27 saves in the victory.
Kaspars Daugavins put Ottawa in front 4:55 into the second period and Daniel Paille tied it with 1:22 to go in the middle stanza.
Ottawa had won its previous three games and Boston had dropped two straight. The Bruins have won 12 of their last 13 meetings with the Senators.
Lehner stopped 27 shots for Ottawa, which dropped five points behind Boston for second place in the Northeast Division.