Rantanen made his professional hockey debut with TPS Turku of Liiga, the top league in Finland, during the 2012-13 season and had three points (two goals, one assist) in 15 games as a 16-year-old forward. He entered 2014-15 as the only 17-year-old to play his third season with TPS and finished with 28 points (nine goals, 19 assists) in 56 games while serving as an alternate captain.

NHL Central Scouting rated Rantanen as the top European skater available in the 2015 NHL Draft, and he was chosen by the Colorado Avalanche in the first round (No. 10). He opened the 2015-16 season with Colorado and made his NHL debut on Oct. 8, 2015, but was sent to San Antonio of the American Hockey League after two weeks. Rantanen had a goal and an assist in his AHL debut and went on to share the Dudley (Red) Garrett Memorial Award, given to the league's outstanding rookie, with Providence forward Frank Vatrano. Rantanen led all AHL rookies in scoring with 60 points (24 goals, 36 assists) and a plus-20 rating in 52 games.

With five points (two goals, three assists) in seven games, Rantanen also helped Finland win a gold medal at the 2016 IIHF World Junior Championship.

Rantanen made the Avalanche for good in 2016-17. He had an assist against the Minnesota Wild on Nov. 5, 2016, scored his first NHL goal six days later against the Winnipeg Jets and finished with 20 goals in 75 games to lead the last-place Avalanche.

The Avalanche rebounded to make the Stanley Cup Playoffs in 2017-18, and Rantanen's improvement was a big reason. He finished with 84 points (29 goals, 55 assists) in 81 games, finishing second on the team in scoring behind linemate Nathan MacKinnon (97 points).

NOTES & TRANSACTIONS

• AHL All-Rookie Team (2016)
• AHL Second All-Star Team (2016)
• Dudley "Red" Garrett Memorial Award (AHL Rookie of the Year) (2016)
• Played in NHL All-Star Game (2019)