[4] Jantunen competed for HBA-Märsky in Finnish second-tier Koripallon I-divisioona from 2016 to 2019, while attending the Helsinki Basketball Academy.

[5] In October 2018, Jantunen committed to Utah over offers from Loyola Marymount, Oregon State, Georgia Tech, and Stanford.

[11] At the end of the season, he won the BNXT League Sixth Man of the Year award, as well as the Belgian championship with Oostende.

[18] Jantunen and Paris became the LNB Pro A Leaders Cup champions in February 2024, by defeating Nanterre 90–85 in the final.

On 4 October 2024, Jantunen debuted in the EuroLeague with Paris, recording nine points, six rebounds and two steals, in a 80–77 defeat against Crvena zvezda in the season opening match.

He was named in the Finland U17 roster at the 2016 FIBA U17 World Cup in Zaragoza, Spain, where he averaged 5.6 points and 6.1 rebounds in seven games.