Määttä played for the IIHF World U20 Championships in 2011, 2012, and 2013. In the 2011 edition, he became the youngest-ever Finnish player to participate in the under-20 tournament. In a game against Canada, Määttä was injured from a hit by Boone Jenner, suffering a concussion, thus ending his tournament. Määttä returned to the international scene to play at the 2011 IIHF World U18 Championships, when he served as an assistant captain for Finland's under-18 team. He was selected by the coaches as one of the Finns' top three players at the tournament, alongside Joel Armia and Miikka Salomäki.