Anton Forsberg stopped 22 shots as Tim Stützle, Josh Norris, Brady Tkachuk and Alex Formenton scored for the Senators (20-31-5). The Blues (32-17-5) lone goal came from David Perron while Jordan Binnington made 24 saves.
"I think that's the best effort we've had all year," Tkachuk said. "Everyone was buzzing, everybody elevated their game and we really emptied the tank to get the win tonight."
After Tkachuk and Stützle had hit the post, the Sens finally found the breakthrough and it came on the power play at 11:35 of the first. A Tkachuk pass hit the skate of Pavel Buchnevich and bounced goalwards where the puck popped out near Colin White who couldn't convert but Stützle could for his 13th, a new career high.
The Blues would tie the game at 14:43 as after the Sens couldn't clear the puck out of their zone, Perron was alone in front and was able to beat Forsberg low for his 13th of the year.
The Senators restored its lead just 1:24 into the second period as Norris scored his 20th of the season on the power play from his usual spot in the right circle for a 2-1 lead.
Vladimir Tarasenko almost tied the game 59 seconds into the third period but was denied by a spectacular glove save as Forsberg preserved the Senators lead.
"It was a save in the right moment," he said.
Captain Tkachuk pushed the lead to 3-1 at 7:48 of the third as he beat Binnington five-hole for his 19th of the season and his first against his hometown Blues. Formenton scored into the empty-net at 17:13 to make it 4-1.
"It was definitely something I've always wanted to do," Tkachuk said of scoring in St. Louis. "I have a lot of memories here and it was pretty cool and pretty special."
Tonight's victory snapped a five-game losing streak for the Sens and sent the Blues to their fourth straight defeat.
Ottawa is back in action when they open a five-game homestand against Seattle Thursday. Puck drop is 7 p.m.
