Galchenyuk made the Canadiens roster for the 2012–13 season opener, a 2–1 loss to the Toronto Maple Leafs on January 19, 2013.
He was the youngest player to play a full season with the Canadiens since 1984.
He scored his first NHL goal on January 22, 2013, against Scott Clemmensen of the Florida Panthers, tipping-in a shot from Brandon Prust; rookie teammate Brendan Gallagher also recorded his first NHL point, an assist.
Galchenyuk finished the season playing in all 48 games with 9 goals, 18 assists for 27 points and a plus-minus rating of +14, playing primarily on the left wing of the third line.
He finished in the top ten in all three offensive categories among rookies, and was sixth overall in rookie points scoring, helping propel a resurgent Canadiens team to the second-best record in the Eastern Conference.

During the 2013–14 season, Galchenyuk missed six weeks due to a broken hand.
He'd finish the season with 13 goals and 31 points in 65 games.

Galchenyuk scored his first career hat-trick on December 16, 2014, against the Carolina Hurricanes.
The 2014–15 season also marked Galchenyuk's first 20-goal campaign, as he tallied that amount in 80 games.

On July 30, 2015, Galchenyuk (as a restricted free agent) signed a two-year, $5.6 million contract extension with the Canadiens.
The following season, Galchenyuk tied captain Max Pacioretty with 30 goals to lead the team.
Galchenyuk finished second in team scoring (behind Pacioretty) with 56 points.

On July 5, 2017, Galchenyuk signed a three-year, $14.7 million contract extension with the Canadiens.
In the following 2017–18 campaign, Galchenyuk produced 51 points in his sixth season with the Canadiens.
Galchenyuk made the Canadiens roster for the 2012–13 season opener, a 2–1 loss to the Toronto Maple Leafs on January 19, 2013. He was the youngest player to play a full season with the Canadiens since 1984. He scored his first NHL goal on January 22, 2013, against Scott Clemmensen of the Florida Panthers, tipping-in a shot from Brandon Prust; rookie teammate Brendan Gallagher also recorded his first NHL point, an assist. Galchenyuk finished the season playing in all 48 games with 9 goals, 18 assists for 27 points and a plus-minus rating of +14, playing primarily on the left wing of the third line. He finished in the top ten in all three offensive categories among rookies, and was sixth overall in rookie points scoring, helping propel a resurgent Canadiens team to the second-best record in the Eastern Conference. 
During the 2013–14 season, Galchenyuk missed six weeks due to a broken hand. He'd finish the season with 13 goals and 31 points in 65 games.
Galchenyuk scored his first career hat-trick on December 16, 2014, against the Carolina Hurricanes. The 2014–15 season also marked Galchenyuk's first 20-goal campaign, as he tallied that amount in 80 games.
On July 30, 2015, Galchenyuk (as a restricted free agent) signed a two-year, $5.6 million contract extension with the Canadiens. The following season, Galchenyuk tied captain Max Pacioretty with 30 goals to lead the team. Galchenyuk finished second in team scoring (behind Pacioretty) with 56 points.
On July 5, 2017, Galchenyuk signed a three-year, $14.7 million contract extension with the Canadiens. In the following 2017–18 campaign, Galchenyuk produced 51 points in his sixth season with the Canadiens.
Alex Galchenyuk