Cole Reinhardt, Matthew Wedman, Max Guenette and Mark Kastelic scored for Ottawa. In goal, Mads Sogaard played the first two periods while Leevi Merilainen got the third frame.
Joe Vrbetic started between the pipes for Montreal as William Trudeau, Mattias Norlinder and Jan Mysak had goals.
"That was a bit of a scrambly game but you've got to get through it and I'm happy we won," Shane Pinto said. "Everyone wants to prove what they can do and just from the get-go you could see guys wanted it and you could see at the end of the third that we wanted it more than them and that's why we won."
Two quick strikes from the Senators in the final frame gave them the victory as Guenette and Kastelic scored 1:32 apart.
Guenette made it 3-3 as after Ben Roger's shot was saved, Vrbetic couldn't locate the rebound and the puck popped loose at the side of the net where the Sens d-man found the equalizer.
Kastelic would give Ottawa the lead at 16:09 as he turned and spun out front to beat Vrbetic low to restore Ottawa's advantage and one they wouldn't relinquish.
"We were on the penalty kill and they scored a nice goal so the guys were a little worried but obviously we came back and got one from Guenette and then another from Kastelic and the guys were pretty fired up," Reinhardt said. "It's pretty cool to see, even though it was just a rookie tournament, how guys were emotionally invested, guys were in this game and how excited everyone was to play."
Reinhardt, who was wearing an 'A' on his jersey, opened the scoring for the Sens at 9:05 as Guenette's feed from behind the goal line found a surging Reinhardt who buried it for a 1-0 lead.
Trudeau tied the game with 2:31 left in the first as he scored on a wraparound.
Ottawa re-took the lead at 11:07 of the middle frame as Wedman scored on a short-handed breakaway as the puck bounced over Norlinder's stick in the offensive zone and the 22-year-old made no mistake against Vrbetic.
Norlinder would get his revenge though as he tied the game with just 46.7 seconds left in the second to make it 2-2 after an errant Ottawa pass.
Montreal grabbed its first lead of the game with 8:21 to play as after Merilainen had been penalized for playing the puck in the trapezoid, Mysak fired in a one-timer for a 3-2 Canadiens advantage before the Senators' rapid finish gave them the victory.
"We talked all week about execution but at the same time being invested for 60 minutes," Senators rookie head coach Troy Mann said. "They were able to get that power play goal but I thought we had some great push back following that and responded with two goals."
Shane Pinto wore the 'C' for Ottawa while Jacob Bernard-Docker wore the other 'A'.
"It was pretty cool," Pinto said of being named captain tonight. "I know it's just a rookie game but I found it pretty cool. I took pride in that and I'm happy we got the win."
Ottawa and Montreal's rookies will square off again Monday at 4:05pm in Brossard, QC.
