In October 2018, he became the first player born in the 2000s to play in one of the major professional sports leagues in the United States and Canada, when he appeared on the Canadiens opening night roster.

Having progressed through the junior ranks and showing offensive potential, Kotkaniemi made his SM-liiga debut with Ässät to start the 2017–18 season, on 9 September 2017 where he scored his first professional goal.

He surpassed fellow Finn Patrik Laine, who, at 18 years, 183 days, recorded a hat trick for the Winnipeg Jets in a 5–4 win against the Toronto Maple Leafs on 19 October 2016.

Seven days later, the Canadiens decided not to match the offer sheet, sending Kotkaniemi to Carolina with Montreal receiving their first and third-round picks in the 2022 NHL Entry Draft as compensation.

Kotkaniemi and Carolina had been rumored to be inking a lengthy contract worth less than his offer sheet agreement since the initial deal was signed.

[21] Kotkaniemi made his junior international debut for Finland at the 2017 World U18 Championships in Slovakia, he amassed 6 points in 7 games to help claim a silver medal.

He returned for the following 2018 World U18 Championships in Russia, centering the top-line in recording 9 points in 7 tournament contests, helping Finland secure the gold medal over United States.