Having ended his junior career, Barré-Boulet attended the Tampa Bay Lightning's rookie and main training camp, before he was reassigned to AHL affiliate the Syracuse Crunch to begin the 2018–19 season. Barré-Boulet scored the game-winning goal in his professional debut on October 6 and went on to be the top-scoring rookie in the AHL during the regular season with 68 points in 74 games for the Crunch, placing sixth in the league's overall scoring race. He earned a share of the Willie Marshall Award as the league co-leader with 34 goals, alongside teammate Carter Verhaeghe. He won the AHL's Dudley "Red" Garrett Memorial Award as the league's most outstanding rookie. He was an all-star during the 2019–20 season, putting up 40 points in 43 games before the All-Star break.
