Giroux, who also had an assist, signed a three-year, $19.5 million contract ($6.5 million average annual value) with Ottawa on July 13.
Alex DeBrincat and Shane Pinto also each had a goal and an assist, and Mathieu Joseph had two assists for the Senators. Anton Forsberg made 21 saves.
Nick Suzuki had two assists in his preseason debut, and Sam Montembeault made 18 saves for the Canadiens, who have lost their first six preseason games (0-5-1).
DeBrincat gave Ottawa a 1-0 lead at 3:13 of the first period on the power play, a one-timer from Giroux for his third preseason goal.
Giroux extended it to 2-0 at 5:52 of the second period, tapping in a pass from DeBrincat.
Joel Armia cut it to 2-1 at 8:20, and David Savard scored 44 seconds later at 9:04 to tie it 2-2.
Tyler Motte scored his third goal of this preseason to give the Senators a 3-2 lead at 11:10, and Pinto restored the two-goal lead for the Senators with his fourth preseason goal at 18:53.
Cole Caufield cut it to 4-3 on the power play at 14:57 of the third period, but Brady Tkachuk scored his first goal of the preseason 1:11 later at 16:08 to give Ottawa a 5-3 lead.
Arber Xhekaj had his wrist shot go off Joseph’s hand at 16:36 for the 5-4 final.
Blues 4, Wild 2: Pavel Buchnevich had a goal and an assist for the St. Louis Blues in a 4-2 win against the Minnesota Wild at Enterprise Center in St. Louis.
Jordan Binnington stopped all 11 shots he faced in the first two periods for the Blues. Joel Hofer made 15 saves in relief.
Tyson Jost and Mason Shaw each had a goal and an assist for the Wild. Filip Gustavsson made 23 saves.
Buchnevich scored on a one-timer from the right circle 1:05 into the first period to give St. Louis a 1-0 lead.
Jake Neighbours’ power-play goal from the left circle made it 2-0 at 11:42.
Nathan Walker scored from the slot to make it 3-0 at 10:18 of the second period.
Shaw cut it to 3-1 on a rebound at 6:58 of the third period, but Klim Kostin scored at 13:57 for the Blues for a 4-1 lead.
Jost scored at 19:19 for the 4-2 final.
Islanders 4, Flyers 3 (OT): Noah Dobson scored with eight seconds left in overtime to give the New York Islanders a 4-3 win against the Philadelphia Flyers at Wells Fargo Center in Philadelphia.
Josh Bailey had a goal and two assists, and Adam Pelech had two assists for the Islanders, who agreed to an eight-year contract with center Mathew Barzal earlier Tuesday. Ilya Sorokin made 30 saves.
Scott Laughton had a goal and an assist, and Samuel Ersson saved all 15 shots he faced in 29:17 for the Flyers in their final preseason game (1-4-1). Troy Grosenick made 14 saves in relief.
Laughton scored on a short-handed breakaway at 2:09 of the second period to give Philadelphia a 1-0 lead.
Bailey tied it 1-1 with a power-play goal at 16:13, and Casey Cizikas scored short-handed at 18:22 to make it 2-1.
Owen Tippett scored at 5:05 of the third period to tie it 2-2, and Tanner Laczynski put the Flyers in front 3-2 at 8:53.
Anders Lee tied it 3-3 for the Islanders at 12:19.
Video: Dobson scores in OT to lift Islanders to 4-3 victory
Sabres 4, Hurricanes 2: Tage Thompson and Dylan Cozens each had a goal and an assist for the Buffalo Sabres in a 4-2 win against the Carolina Hurricanes at KeyBank Center in Buffalo.
Vinnie Hinostroza and JJ Peterka scored, and Craig Anderson made 23 saves for Buffalo.
Jamieson Rees and Malte Stromwall scored, and Pyotr Kochetkov made 26 saves for Carolina in its preseason finale (4-1-0).
The Sabres led 3-0 after the first period.
Hinostroza made it 1-0 at 6:59 on Buffalo’s 10th straight shot to start the game.
Thompson skated around Hurricanes defenseman Ethan Bear and scored over Kochetkov’s shoulder to make it 2-0 at 10:52.
Cozens pushed the lead to 3-0 at 18:16 when he intercepted a drop pass in his own zone and scored on a short-handed breakaway.
Rees cut it to 3-1 at 1:45 of the third period, and Stromwall scored 19 seconds later at 2:04 to pull Carolina within 3-2 before Peterka scored into an empty net at 18:40 for the 4-2…
Read More: Oct. 4: NHL Preseason Roundup