Entering the 2014–15 season as the only 17-year-old to play in his third year with TPS, Rantanen signed a two-year extension to remain with the Liiga outfit on 9 October 2014.

He also featured for TPS junior club in the post-season recording 14 points in 7 contests to help capture the championship and was given Best Player honours for his efforts.

In his National Hockey League (NHL) draft-eligible year, Rantanen was rated as the top European skater at season's end.

[5] After attending his first Avalanche training camp, and impressing in the preseason, Rantanen was announced to have made the opening night roster for the 2015–16 season as an 18-year-old on 6 October 2015.

[8] He was used in a depth role and played limited minutes over six scoreless games with Colorado before being sent to the team's American Hockey League (AHL) affiliate, the San Antonio Rampage, on 22 October 2015.

[10] He continued his scoring pace with the Rampage, leading the club in all offensive categories before he was selected to the AHL All-Star Game as the second-youngest participant in the event's modern history.

[12] In finishing sixth in overall scoring, he earned a selection to the Second All-Star Team and shared the Dudley "Red" Garrett Memorial Award alongside Frank Vatrano as the AHL's rookie of the year.

Entering the 2024–25 season and in the final year of his contract before he was set to become an unrestricted free agent, Rantanen and the Avalanche hit an impasse when it came to contract extensions; Rantanen was reportedly seeking an annual salary in the range of $14 million, while the Avalanche were unwilling to offer more than $12.6 million a year (the salary of forward Nathan MacKinnon).

[26][27] While adding a stabilising veteran presence, Rantanen played a supporting role throughout the round-robin stage before stepping up his production in the Semi-final against Sweden and scoring in the Final against Russia to help Finland claim the gold medal and cap his junior career.