In the
2016–17 season, Dahlin scored his first
Swedish Hockey League (SHL) goal with
Frölunda HC on 12 November 2016 in a game against
Karlskrona HK. in 2018. Leading up to the
2018 NHL entry draft, Dahlin was widely considered the top prospect. He was described as a smart two-way defenceman with a great set of tools, including skating, puck handling, vision, intelligence and shot. Dan Marr of the
NHL Central Scouting Bureau described him as an exceptionally talented prospect that would be able to impact his team's fortune in a way similar to his countrymen
Erik Karlsson and
Victor Hedman. On 22 June 2018, Dahlin was drafted first overall by the
Buffalo Sabres. 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 9 July, 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 4 October in a 4–0 loss to the
Boston Bruins. Dahlin recorded his first career NHL goal on 13 October 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. On 22 September 2021, Dahlin signed a three-year, $18 million contract with the Sabres. He enjoyed a breakout
2021–22 season, posting new career highs in goals, assists and points and was named to the
2022 NHL All-Star Game. Dahlin opened the
2022–23 season by setting an NHL record for goals by a defenceman in consecutive opening games, at four, as part of a resurgent Sabres team. Dahlin was later named to his second consecutive
All-Star Game, replacing injured teammate
Tage Thompson. On 9 October 2023, Dahlin signed an eight-year, $88 million contract extension with the Sabres. Dahlin was named
captain of the Sabres on 26 September 2024, succeeding
Kyle Okposo. On 27 January 2026, Dahlin scored his first career
hat-trick in a 7–4 win over the
Toronto Maple Leafs. ==International play==