Aittokallio returned for the 2012 World Junior Championships, and was Finland's starting goaltender, with Christopher Gibson serving as his back-up. In Finland's semifinal against Sweden, Aittokallio made 56 saves, but ultimately, Finland lost 2–3 in a shootout. Aittokallio was named Finland's player of the game with his impressive performance. Following that loss, they played in the bronze medal game against Canada, which they lost 0–4. However, Finland recorded their best performance in the World Juniors since 2006 with their 4th-place exit and Aittokallio was subsequently selected in Finland's top three players for the tournament.