Stamatis was a member of the Greek junior national teams. With the junior national teams of Greece, he played at some of the following tournaments: the 2011 FIBA Europe Under-16 Championship, the 2012 FIBA Europe Under-16 Championship, the 2013 FIBA Europe Under-18 Championship, the 2014 FIBA Europe Under-18 Championship. He also played at the 2015 FIBA Under-19 World Cup, the 2015 FIBA Europe Under-20 Championship, and the 2nd division 2016 FIBA Europe Under-20 Championship Division B, where he won a bronze medal.