Sweden in August 2008. Karlsson made his
Elitserien debut for
Frölunda HC on 1 March 2008, recording 8:48 of ice time during a sold out home game in
Scandinavium against league leaders
HV71. Karlsson scored the game-winning goal, assisted by
Tomi Kallio and
Magnus Kahnberg, with a
slap shot in overtime during a
man advantage. The win secured a playoff berth for Frölunda with three games remaining in the regular season. Karlsson finished the season with Frölunda's J20 team playing in the
J20 SuperElit playoffs, where Frölunda took home the
Anton Cup when they won, two games to one, against
Brynäs IF's J20 team in the Swedish Junior Ice Hockey Championship final. Prior to the
2008 NHL entry draft, Karlsson was ranked fourth among European skaters by the NHL's Central Scouting Service. He was ultimately drafted 15th overall by the
Ottawa Senators in front of their hometown fans at
Scotiabank Place in
Ottawa. The selection was made by Ottawa's
captain Daniel Alfredsson, a native of
Gothenburg, who played for Frölunda before entering the NHL. Then-Ottawa general manager
Bryan Murray traded Ottawa's first-round pick, 18th overall, and their third-round pick in
2009 to the
Nashville Predators in exchange for the Predators' first-round pick, 15th overall, to ensure that no other team would select Karlsson before them. A few weeks before the
2008–09 Elitserien season premiere, Frölunda announced that Karlsson was brought up to the senior team as a regular roster player.
Ottawa Senators (2009–2018) in November 2009 In September 2009, Karlsson attended the Senators' training camp ahead of the
2009–10 season. On 29 September 2009, the Senators announced that Karlsson had made the team's NHL roster. Karlsson made his NHL debut on 3 October, against the
New York Rangers, and recorded his first NHL point, an assist. After struggling in nine regular season games with Ottawa, he was assigned to the team's
American Hockey League (AHL) affiliate, the
Binghamton Senators. On 27 November 2009, exactly one month after being sent down, Karlsson was recalled from Binghamton. He would score his first NHL goal against the
Minnesota Wild's
Niklas Bäckström in a 4–1 win for Ottawa on 19 December 2009, and would remain in the NHL for the remainder of the season and play in all of Ottawa's
2010 Stanley Cup playoff games. Karlsson was selected to participate in the
2011 NHL All-Star Game, set for 30 January 2011, at the
RBC Center in
Raleigh, North Carolina, having recorded 25 points in 41 games. The
2011–12 season saw Karlsson continue his development. On 16 December 2011, with his third assist of the night, Karlsson registered his 100th regular-season NHL point (in 168 games) in a game against the rival
Pittsburgh Penguins. Karlsson was the NHL's leading vote-getter in All-Star voting, receiving 939,951 fan votes and becoming one of four Senators players selected to partake in the
2012 NHL All-Star Game. Karlsson finished the season as the leading scorer among NHL defencemen, leading second-place
Dustin Byfuglien and
Brian Campbell by 25 points. Karlsson was being mentioned as a
James Norris Memorial Trophy candidate, if not the favourite for the award. During the season he set a new Senators franchise record for points in a season by a defenceman (previously held by
Norm Maciver) with 78. On 19 June 2012, Karlsson signed a reported seven-year, $45.5 million contract with the Senators. The following day, Karlsson was announced as the winner of the James Norris Trophy as the NHL's best defenceman, beating
Zdeno Chára of the
Boston Bruins and
Shea Weber of the
Nashville Predators. He became the second Swede after seven-time winner
Nicklas Lidström to win the award, joining the ranks of
Hall of Famers Bobby Orr and
Denis Potvin as the only players to win the award under age 23. in November 2012 during the
2012–13 NHL lockout During the
2012–13 NHL lockout, Karlsson signed with
Jokerit of the Finnish
SM-liiga. He received a one-game suspension for allegedly throwing his stick at a referee following a game on 8 December 2012. He finished his stint in Jokerit with 9 goals and 25 assists (34 points) in 30 games, leading all defencemen in scoring. Once the lockout ended and the
season commenced in January 2013, Karlsson promptly recorded a goal and two assists as Ottawa defeated the
Winnipeg Jets 4–1 in their season opener. On 13 February 2013, Karlsson's
Achilles tendon was lacerated when Pittsburgh Penguins forward
Matt Cooke had his skate land on the back of Karlsson's left leg, requiring surgery and taking him out of Ottawa's lineup indefinitely. Cooke had been suspended several times previously for much-criticized incidents resulting in injury to opposing players, but was not suspended for this incident. At the time of the injury, Karlsson led all NHL defencemen with six goals. Though initial estimates had him out of the lineup for four-to-six months, Karlsson returned to the Ottawa lineup against the
Washington Capitals on 25 April 2013, ten weeks to the day after the injury occurred. The Senators made the
2013 Stanley Cup playoffs but were eliminated by Pittsburgh in the Eastern Conference semifinals, with Karlsson registering one goal and seven assists in ten games. Karlsson appeared in all 82 games during the
2013–14 season and finished the season with 20 goals, 54 assists and 74 points, resulting in Karlsson being the first defenceman since
Brian Leetch in
2000–01 to have at least 20 goals and 50 assists in the same season. He passed
Steve Duchesne for franchise record for most goals in a season by a defenceman with 20. However, the Senators would fail to qualify for the
2014 playoffs. On 2 October 2014, the Senators organization announced that Karlsson would serve as the ninth captain in the team's modern history, replacing the recently traded
Jason Spezza. In his first season as team captain, Karlsson led all NHL defencemen in points for the third time in four seasons, including a career-high 21 goals. He also played in all 82 of Ottawa's games for a consecutive season and ranked third in the NHL in total ice time (2,234:55) and average ice time (27:15) to carry the Senators to a 23–4–4 record on the way to an unlikely playoff spot. On 24 June 2015, it was announced Karlsson won his second Norris Trophy, beating out fellow nominees
Drew Doughty and
P. K. Subban. Karlsson appeared in all 82 games for the third-straight season during the
2015–16 season and led the league in assists with a career high 66 assists and set a career high in points (82), finishing fourth in the league in scoring alongside
San Jose Sharks forward
Joe Thornton. With his 81st point, Karlsson broke the record for most points in a single season by a Swedish defenceman, which was previously set by Nicklas Lidström during the
2005–06 season. Karlsson was also the first defenceman since
Paul Coffey in the
1985–86 season to finish in the top five in scoring and the first since Bobby Orr in the
1974–75 season to lead the league in assists. Karlsson's performance earned him his third Norris Trophy nomination, though Drew Doughty won the trophy with Karlsson finishing in second place in vote totals. During the
2016–17 season, Karlsson set a Senators record on 4 March 2017, when he appeared in his 312th consecutive game, breaking the previous set by defenceman
Chris Phillips. However, he would miss his first game in almost four years in late March 2017 after sustaining an injury from blocking a shot during a game against the
Philadelphia Flyers, ending his consecutive game streak at 324 games. Karlsson finished the regular season in third place among defencemen in points and second place among defencemen in assists and blocked shots, earning him his fourth Norris Trophy nomination. Karlsson would finish second in voting, with the award going to
Brent Burns of the
San Jose Sharks. Karlsson's performance continued into the
2017 playoffs, helping the Senators reach the conference finals, the first time the team had done so since
2007. Although the Senators would be eliminated by the Pittsburgh Penguins in seven games losing the deciding seventh game in double overtime, one win and goal short of reaching the
2017 Stanley Cup Final, Karlsson was praised for his performance during the Senators' playoff run and how he continued playing despite suffering two hairline fractures in his left heel. Karlsson would also set a playoff team record for most assists and points for a defenceman in the playoffs, breaking the previous record set by
Wade Redden in the 2006 playoffs (eight assists, 10 points). And despite being eliminated in the conference finals, Karlsson received a
Conn Smythe Trophy vote without playing a game in the Stanley Cup Final. Shortly after the Senators were eliminated from the playoffs, Karlsson had surgery to repair torn tendons in his left foot, resulting in him missing the beginning of the
2017–18 season. Karlsson's productivity slightly dwindled in this season, finishing the season with 9 goals, 53 assists and 62 points in 71 games, partly due to coping with injuries and the loss of his child towards the end of the season. Despite his lower-than-average performance, Karlsson moved into third place on the franchise's all-time points list (with 492 points) on 8 February 2018 after a 4–3 win over the Nashville Predators. The Senators also struggled during the season, finishing the season in 30th place in the league. In the midst of a rebuild, the Senators attempted to trade Karlsson before the
NHL trade deadline, as his seven-year contract was ending in 2019 and the organization was uncertain about re-signing him, though a deal could not be reached before the deadline. However, the day after the deadline, Karlsson expressed his interest in staying in Ottawa and said that he never requested a trade.
San Jose Sharks (2018–2023) On 13 September 2018, Karlsson (along with
Francis Perron) was traded to the San Jose Sharks in exchange for
Chris Tierney,
Dylan DeMelo,
Josh Norris,
Rūdolfs Balcers, the Sharks'
2020 first-round pick, a
2019 second-round pick and a conditional
2021 second-round pick. He scored his first goal with the Sharks on 18 November in a 4–0 win over the
St. Louis Blues. After a slow start to the
2018–19 season, Karlsson became the fifth defenceman in league history to have at least one assist in 14 consecutive games following a 7–2 victory over the
Edmonton Oilers on 8 January 2019. On 23 December, Karlsson was suspended for the first time in his career for two games for an illegal check to the head of
Los Angeles Kings player
Austin Wagner. A groin injury resulted in Karlsson missing many games near the end of the season before returning for the last game of the season against the
Colorado Avalanche. He would finish the season with 45 points in 53 games, his lowest point tally since the 2012–13 season. Karlsson assisted San Jose in reaching the Western Conference finals during the
2019 playoffs, though he sustained another groin injury in a game against the St. Louis Blues, resulting in him missing San Jose's final game of the playoffs when the team lost 5–1 in game 6. He would finish the playoffs with 16 points in 19 games. On 31 May, he underwent groin surgery. During the 2018–19 campaign Karlsson set a Sharks franchise record for most consecutive games with a point with 14. On 17 June 2019, Karlsson signed a new eight-year, $92 million contract to remain with the Sharks, with an annual value of $11.5 million. His new contract made Karlsson the highest-paid defenceman in league history and the third highest-paid player in the league at the time, behind
Auston Matthews ($11.6 million annually) and
Connor McDavid ($12.5 million annually). He broke his thumb on 14 February 2020, in a game against the
Winnipeg Jets. He was placed on injured reserve and would miss the remainder of the 2019–20 season. During the
2022–23 NHL season Karlsson scored his first NHL career
hat-trick on 1 November 2022 in a 6–5 shootout loss to the
Anaheim Ducks. By 3 February 2023, he was leading all defencemen in points, having accumulated 66 by that time, and was among the leading candidates for the Norris Trophy. On 10 April, Karlsson became the first defenceman to score 100 points in a season since
Brian Leetch accomplished it during the
1991–92 season, a feat only five other defencemen have accomplished in league history. Karlsson also became the first Swedish defenceman to accomplish the feat. He finished the season with 101 points (25 goals, 76 assists), a career high in all three categories. His achievements that year attracted wide recognition, and Karlsson was for the first time in his career named a finalist for the
Ted Lindsay Award, voted on by the
National Hockey League Players' Association for the league's most outstanding player. He was also named a Norris Trophy finalist for the fifth time. He won the award and was named to the NHL First All-Star team on 26 June 2023. During the season, Karlsson also set an NHL record for defencemen by scoring or assisting on 43.35 percent of San Jose's total goals (101 of 233, excluding shootout-deciding goals), while also setting a new Sharks franchise record for points in a season by a defenceman (previously held by
Brent Burns) with 101. In 2023, he was awarded the
Victoria Award.
Pittsburgh Penguins (2023–present) On 6 August 2023, Karlsson was traded to the
Pittsburgh Penguins in a three-team deal also involving the
Montreal Canadiens, with Pittsburgh also receiving forward
Rem Pitlick from the Canadiens, as well as forward Dillon Hamaliuk and a
2026 third-round draft pick from San Jose, while Pittsburgh sent out a
2024 first-round draft pick, forward
Mikael Granlund, and defenceman
Jan Rutta to San Jose (with San Jose also receiving forward Mike Hoffman from Montreal in the trade). Pittsburgh also sent a
2025 second-round draft pick, defenceman
Jeff Petry, goaltender
Casey DeSmith and forward
Nathan Légaré to Montreal. In the
2023–24 season, Karlsson scored 11 goals and 56 points in 82 games played in his first season with the Penguins. Karlsson finished the
2024–25 season with 11 goals and 52 points. ==Style of play==