Last season, the 18-year-old divided his time between Karpat's Under-18 and Under-20 teams in SM-sarja, Finland's junior hockey leagues. At the Under-20 level, he played 31 games and scored 13 points.
With the Under-18 squad, Pieniniemi took part in three regular-season games, tallying seven points. He also played an important role in the playoffs, contributing six points across seven games, helping his team win the championship.