On 22 June 2018, Dahlin was drafted first overall by the Buffalo Sabres in the Draft. He was the first Swedish player to be drafted first overall since Mats Sundin in 1989, and the second Swede overall to accomplish the feat. On July 9, he signed a three-year, entry-level contract with the Sabres. Dahlin participated at the Sabres' training camp and made the opening night roster for the 2018–19 season. He made his NHL debut on October 4 in a 4–0 loss to the Boston Bruins. Dahlin recorded his first career NHL goal on October 13 in a 3–0 win over the Arizona Coyotes. On 9 February 2019, after a win over the Detroit Red Wings, Dahlin became the fifth defenceman in NHL history to record 30 points before age 19. Following his rookie season, Dahlin was named as one of the three finalists for the Calder Memorial Trophy as the top rookie in the NHL.