FIBA Europe
With Greece's junior national teams, Giannopoulos played at the 2005 FIBA Europe Under-16 Championship and the 2006 FIBA Europe Under-18 Championship. He won the silver medal at the 2007 FIBA Europe Under-18 Championship. He also played at the 2008 FIBA Europe Under-20 Championship, and won the gold medal at the 2009 FIBA Europe Under-20 Championship.