Trailing 5-3 going into the final period, Kovalev scored on a rush down the left side at 3:51, Samsonov banged in Andrei Markov's pass at 15:16 and Kovalev had the sell-out crowd of 21,273 on its feet as he got the game-winner with 1:52 left to play.
The left Montreal at 3-5 and Ottawa at 5-4 as both clubs completed their pre-season schedules.
Kovalev and Samsonov were playing with their new centre Tomas Plekanec after Mike Ribeiro was traded to Dallas just before the game for defenceman Janne Niinimaa.
Plekanec assisted on two of the third-period goals. His drop pass to Kovalev for a spin-around backhand from the slot produced the winning goal.
Mike Johnson, Michael Ryder and Sheldon Souray also scored for the Canadiens, who were beaten 5-2 in Ottawa on Thursday night.
Dany Heatley, with his fifth of the pre-season, and Andrej Meszaros scored in the first frame and Peter Schaefer, Cristoph Schubert and Chris Neal had goals in the second for Ottawa.
The Senators scored 34 goals in nine pre-season games, best in the Eastern Conference.
Montreal outshot Ottawa 30-25 and neither Montreal's Cristobal Huet nor Ottawa's Martin Gerber was sharp in goal.
After Johnson tapped in Radek Bonk's rebound 2:13 into the game, Ottawa scored on two of its first three shots on Huet as Heatley shot into an open side at 5:08 and Meszaros scored on a point shot at 35 seconds later.
Ryder tied it at 7:52 on a backhand shot from the slot.
Schaefer and Souray exchanged goals early in the second frame and Schubert put Ottawa ahead on a power play at 5:27 on a low shot from the blue line.
Neil got his fourth of the pre-season at 6:22 when he pushed a loose puck on a rush past Huet.
Kovalev scored on a blast from the left wing on a rush 3:51 into the third period.
The Senators killed off a two-man Montreal advantage for 1:23 midway through the final period.