The next season, as he turned 16 during the summer, Määttä was automatically promoted to JYP under-18 team, but was swiftly promoted after only one game to the under-20 team. While splitting his time between JYP U20 and the D Team in the Mestis (second division) classification—the second-highest level of ice hockey in Finland—Määttä once again found time to participate on Finland's national teams, playing for both the under-18 and -20 teams. At the under-18 tournament, he led all Finnish defenceman in scoring with four points and helped Finland to a fifth-place finish. Määttä did not perform nearly as well at the 2011 World Junior Championships, however, going pointless in six games, though he became the youngest player ever to play for Finland's World Junior team, a record that has since been broken by Aleksander Barkov. At the end of the season, Määttä returned to JYP's U20 team en route to an eventual league championship. Määttä finished with five points in 12 games.