Rantanen made the most of his season as an 18-year-old on TPS' senior SM-Liiga squad. He was second on his team in scoring with nine goals and 28 points playing against men on a team that finished 13th in a 14 team league. He led all U-20 players in scoring across Finland's top circuit. When he joined TPS' U-20 squad for their playoff campaign he was dominant, recording six goals and 14 points in just seven games.
At the international level, Rantanen has been a staple for Finland since reaching the U-16 level. Despite a disappointing showing from the Finnish side at this past year's World Juniors, he was a top performer.