Larsson was selected in the first round, fourth overall, by the New Jersey Devils in the 2011 NHL Entry Draft.
Seen as a move towards rebuilding the Devils defence corps, Larsson was rated by the NHL Central Scouting Bureau as the top European-based prospect available in the draft.
Larsson, who has been compared to the Detroit Red Wings' defenceman Nicklas Lidström, signed a three-year, entry-level contract with no bonuses at the NHL maximum of US$925,000 per year with the Devils on 15 July 2011, due to general manager Lou Lamoriello's disapproval of performance bonuses.
He made the team in training camp and became the first 18-year-old defenceman to dress for the Devils in the regular season since Scott Niedermayer in the 1991–92 season, only the sixth under-20 defenceman to play for the Devils, and the first 18-year-old defenceman in the NHL since 2008, when Drew Doughty, Zach Bogosian and Luke Schenn all made their NHL debuts.

Larsson's first NHL goal was scored on 11 November 2011 on a slapshot from the right slot against Tomáš Vokoun of the Washington Capitals.
He was selected to the NHL All-Star Rookie SuperSkills Competition on 12 January 2012, along with teammate Adam Henrique; however, he chose to pull out due to a minor injury.
He was replaced by Montreal Canadiens rookie defenceman Raphael Diaz.
On 1 May 2012, Larsson scored his first Stanley Cup playoff goal against the Philadelphia Flyers in Philadelphia in his first career playoff game, becoming the first Devils rookie defenceman to score a goal in his first playoff game.

Following the 2014–15 season, Larsson became a restricted free agent under the NHL Collective Bargaining Agreement (CBA).
The Devils made him a qualifying offer to retain his NHL rights and on 5 July 2015, he filed for salary arbitration under the CBA.
On 25 July, he signed a new six-year, $25 million contract with the Devils.
Larsson was selected in the first round, fourth overall, by the New Jersey Devils in the 2011 NHL Entry Draft. Seen as a move towards rebuilding the Devils defence corps, Larsson was rated by the NHL Central Scouting Bureau as the top European-based prospect available in the draft. Larsson, who has been compared to the Detroit Red Wings' defenceman Nicklas Lidström, signed a three-year, entry-level contract with no bonuses at the NHL maximum of US$925,000 per year with the Devils on 15 July 2011, due to general manager Lou Lamoriello's disapproval of performance bonuses. He made the team in training camp and became the first 18-year-old defenceman to dress for the Devils in the regular season since Scott Niedermayer in the 1991–92 season, only the sixth under-20 defenceman to play for the Devils, and the first 18-year-old defenceman in the NHL since 2008, when Drew Doughty, Zach Bogosian and Luke Schenn all made their NHL debuts.
Larsson's first NHL goal was scored on 11 November 2011 on a slapshot from the right slot against Tomáš Vokoun of the Washington Capitals. He was selected to the NHL All-Star Rookie SuperSkills Competition on 12 January 2012, along with teammate Adam Henrique; however, he chose to pull out due to a minor injury. He was replaced by Montreal Canadiens rookie defenceman Raphael Diaz. On 1 May 2012, Larsson scored his first Stanley Cup playoff goal against the Philadelphia Flyers in Philadelphia in his first career playoff game, becoming the first Devils rookie defenceman to score a goal in his first playoff game.
Following the 2014–15 season, Larsson became a restricted free agent under the NHL Collective Bargaining Agreement (CBA). The Devils made him a qualifying offer to retain his NHL rights and on 5 July 2015, he filed for salary arbitration under the CBA. On 25 July, he signed a new six-year, $25 million contract with the Devils.
Adam Larsson