The 6-foot-2, 176-pound defenseman split the 2022-23 season between Karpat's Under-18 and Under-20 teams in SM-sarja, Finland's junior hockey leagues. He played 31 games at the Under-20 level, recording one goal, 12 assists and 13 points. With the Under-18 squad, Pieniniemi played three regular-season games, recording three goals and seven points. He also went on to tally six points (3G-3A) in seven playoff games, helping his team to the U-18 SM-sarja Championship.