A first-round pick by the Islanders (No. 16) in the 2015 NHL Draft, Barzal made his NHL debut at the Washington Capitals on Oct. 15, 2016. He played one more game with New York before he was returned to Seattle of the Western Hockey League.

Barzal was MVP of the WHL playoffs in 2017 after scoring 25 points (seven goals, 18 assists) in 16 games, helping Seattle win the league championship for the first time. He was also the WHL's Western Conference Player of the Year after finishing with 79 points (10 goals, 69 assists) in 41 regular-season games.

He became an impact player with the Islanders in 2017-18. He scored his first NHL goal against the New York Rangers at Madison Square Garden on Oct. 19, 2017, then became the first Islanders rookie to have five assists in one game against the Colorado Avalanche on Nov. 5, 2017. Barzal scored his first NHL hat trick against the Winnipeg Jets on Dec. 23, 2017 and his first NHL overtime goal four nights later against the Buffalo Sabres.

He led the Islanders in points (85) and assists (63) and won the Calder Trophy voted as the top rookie in the NHL.

Barzal led the Islanders in scoring again in 2018-19, when he was named to the NHL All-Star Game and helped New York return to the Stanley Cup Playoffs and sweep the Pittsburgh Penguins in the Eastern Conference First Round. He returned to the NHL All-Star Game in 2019-20 and led the Islanders with 60 points (19 goals, 41 assists) in 68 regular-season games before the season was paused March 12 due to concerns surrounding the coronavirus.

Barzal signed an eight-year contract with the Islanders on Oct. 4, 2022, after his 59 points (15 goals, 44 assists) in 73 games was the fifth straight season atop the team's leaderboard in points. He scored his 100th NHL goal in his 400th game, a 4-1 loss at the Seattle Kraken on Jan. 1, 2023, and finished that season with an Islanders-best 80 points (23 goals, 57 assists).

A native of Coquitlam, British Columbia, Barzal represented Canada at the 2017 IIHF World Junior Championship.