Selected by the Anaheim Ducks in the second round (No. 38) of the 2014 NHL Draft, Pettersson made his NHL debut Feb. 15, 2018, against the Chicago Blackhawks.

The defenseman scored four points (one goal, three assists), had a plus-5 rating and averaged 13:37 of ice time in 22 games.

Pettersson's first NHL point came in his eighth game, when he scored against the Blackhawks on March 4, 2018. He also played in all four Stanley Cup Playoff games when the Ducks lost to the San Jose Sharks in the Western Conference First Round.

Prior to coming to the NHL, Pettersson developed with the Skelleftea hockey program in his native Sweden, climbing the ranks until he made his debut with its first team in the Swedish Hockey League at age 17 during the 2013-14 season. He had a plus-2 rating in 10 games.

Pettersson played for Sweden at the 2014 IIHF World Under-18 Championship, the 2016 IIHF World Junior Championship and the 2019 World Championship. He made the jump to North America in 2017-18 and had 14 points (all assists) in 44 games with San Diego of the American Hockey League before being called up by the Ducks.

The Penguins acquired Pettersson in a trade with the Ducks on Dec. 3, 2018, and he signed a five-year contract with the Penguins on Jan. 28, 2020. He was in the final season of the contract when he was traded to the Vancouver Canucks with Drew O'Connor for Danton Heinen and Vincent Desharnais on Feb. 1, 2025. Pittsburgh also received forward prospect Melvin Fernstrom and a conditional first-round pick in the 2025 NHL Draft.

Pettersson signed a six-year, $33 million contract with the Canucks four days after the trade, general manager Patrik Allvin saying he "has already shown us the type of leadership, poise and character that we want in a top-four defenseman."