Karlsson is the rare player who can change a game every time he steps onto the ice. The San Jose Sharks acquired him in a trade with the Ottawa Senators on Sept. 13, 2018. He signed an eight-year contract June 17, 2019 to avoid becoming an unrestricted free agent and completed his first NHL hat trick at 32 years old in a 6-5 shootout loss to the Anaheim Ducks on Nov. 1, 2022. He joined Nicklas Lidstrom, Zdeno Chara, Ed Jovanovski and Lubomir Visnovsky as NHL defensemen age 32 or older to get a hat trick. On Nov. 27, Karlsson had two assists in a 4-3 win against the Vancouver Canucks to become the first defenseman in Sharks history to get at least 20 points in a calendar month.

To begin 2023, Karlsson had two assists to set a Sharks record 13-game point streak in a 5-2 win against the Chicago Blackhawks at United Center on Jan. 1, and extended the run to 14 games. On April 10, he scored twice to become the sixth defenseman in NHL history to reach 100 points in a season since Brian Leetch had 102 points with the New York Rangers in 1991-92.

Karlsson won the 2022-23 Norris Trophy, voted as the best defenseman in the NHL, for the third time after finishing with 101 points (25 goals, 76 assists). He became the ninth player in NHL history to win the Norris Trophy three times, joining Hockey Hall of Famers Bobby Orr (eight), Doug Harvey (seven), Nicklas Lidstrom (seven), Ray Bourque (five), Paul Coffey, Chris Chelios, Pierre Pilote and Denis Potvin (three each). The win came 11 years after Karlsson's first (2011-12), the longest gap in NHL history.

The next chapter in Karlsson's storied career arrived when the Sharks traded him to the Pittsburgh Penguins in a three-team deal involving the Montreal Canadiens on Aug. 6.

Karlsson's 56 points (11 goals, 45 assists) led Penguins defensemen in 2023-24. He became the eighth-fastest at his position with 600 NHL assists Dec. 30, 2023, ninth-fastest to 800 points (973rd game) and was the 129th to skate 1,000 games when Pittsburgh lost 6-4 to the Boston Bruins at PPG Paints Arena on April 13, 2024. He was named to Sweden's roster for the 4 Nations Face-Off and Team Sweden for the Olympic Winter Games Milano Cortina 2026.

Long establishing himself not only as one of the best defensemen in the NHL and its top players at any position, Karlsson's speed, puck sense and passing are superb, and he's proven to be durable and tough despite playing heavy minutes on a nightly basis. For example, he sustained a 70 percent tear in his left Achilles tendon Feb. 13, 2013, and required surgery, but was back in uniform less than three months later and helped the Senators advance to the second round of the Stanley Cup Playoffs.

Born in Landsbro, Sweden, Karlsson is a product of the Frolunda HC system in his home country. He was selected by the Senators in the first round (No. 15) of the 2008 NHL Draft. The draft was held at Scotiabank Place in Ottawa, with Senators captain and fellow Sweden-born player Daniel Alfredsson taking the stage to announce the selection. Ottawa traded its first-round pick (No. 18) and a third-round selection to the Nashville Predators to move up three spots and secure Karlsson.

Karlsson had a breakout season in 2011-12, his third in the NHL, finishing with 78 points (19 goals, 59 assists) and winning the Norris Trophy, edging Zdeno Chara of the Bruins and Shea Weber of the Predators. In doing so, Karlsson became the second Sweden-born player to win the Norris -- following one of his idols, Nicklas Lidstrom -- and the third player under 23 years old to win the award, joining Bobby Orr and Denis Potvin.

Karlsson scored 20 goals in 2013-14 and 21 in 2014-15, when he won the Norris Trophy for the second time, then tied for fourth in the NHL with 82 points (16 goals, 66 assists) in 2015-16. It was the first time a defenseman finished in the top five in scoring since 1985-86, when Paul Coffey of the Edmonton Oilers was third with 138.

He had 71 points (17 goals, 54 assists) in 2016-17, when he was named an NHL First All-Star Team for the third straight season. Despite playing with an injured left foot that required offseason surgery, Karlsson led the Senators in postseason scoring with 18 points (two goals, 16 assists) in 19 Stanley Cup Playoff games, although Ottawa lost Game 7 of the Eastern Conference Final to Pittsburgh in double overtime.

Though the Senators failed to qualify for the playoffs in 2017-18, Karlsson finished with 62 points (nine goals, 53 assists) despite missing 11 games because of injuries. In his first season with the Sharks, Karlsson helped them reach the 2019 Western Conference Final, when they lost in six games to eventual Stanley Cup champion St. Louis Blues. He had 16 points (two goals, 14 assists) in 19 playoff games after he had 45 (three goals, 42 assists) in 53 games in the regular season.