2010-11: Rasmus Ristolainen made his SM-Liiga debut as a 16-year-old, seeing 6:41 minutes of ice time in a February game for TPS Turku, and spent much of the season with the club's U20 team; also skating for the club's U18 team in the SM-Liiga U18 playoffs. He represented Finland at the U17 World Hockey Challenge. Ristolainen played 27 games with TPS' U20 team and was minus-20 with 12 assists and 30 penalty minutes. TPS reached the U18 finals and In 13 playoff games Ristolainen scored 5 goals with 3 assists and had 8 penalty minutes. He played five games at the WHC and had 1 goal and 4 penalty minutes.
2011-12: Ristolainen made TPS' SM-Liiga team out of camp and appeared in 40 pro games. He represented Finland in the U18 and U20 World Junior Championship tournaments. After averaging over 20 minute of ice time in his first eight games, Ristolainen's ice time decreased a bit but he ended the season with 3 goals and 5 assists and was minus-21 with 78 penalty minutes; averaging 16:33 minutes of ice time. He appeared in two playoff games and was minus-two. In eight games with TPS' U20 team he had 4 assists and was plus-four. With Olli Maatta out of the lineup for Finland's U20 team, Ristolainen played in all seven games, including the bronze medal game, at the U20 WJC. He was plus-one with 3 assists as Finland finished fourth. Ristolainen was an assistant captain for Finland's U18 team and was plus-three with 2 goals, 1 assist and 10 penalty minutes in seven games. Ristolainen was selected by Amur Khabarovsk in the first round (12th overall) of the 2012 KHL Draft.
2012-13: Ristolainen assumed a larger role for TPS in his second SM-Liiga season — skating in 52 games for Turku — and played for Finland at the 2013 World Junior Championship in Ufa. While the season was a disappointment team-wise (TPS missed the playoffs after finishing 13th in the 14-team league), Ristolainen was the team's leading scorer amongst defensemen. He scored 3 goals with 12 assists and was minus-seven with 32 penalty minutes; averaging 21:40 minutes of ice time. In six games for seventh-place Finland at the WJC he scored 2 goals with 4 assists and was plus-five. He joined TPS' U20 team for the league playoffs and scored 2 goals with 1 assist and was minus-five in five games. Ristolainen was ranked third among European skaters in Central Scouting's mid-term rankings prior to the 203 NHL Draft, and fourth in the CSS final rankings.