Aimar first represented Argentina at youth levels. Along with future senior team players Esteban Cambiasso and Juan Román Riquelme he won the 1997 FIFA World Youth Championship and went on to earn 52 senior caps following his debut in 1999, scoring eight international goals in total. He represented Argentina at the 1999 and 2007 editions of the Copa América, at the 2002 and 2006 FIFA World Cups, and at the 2005 FIFA Confederations Cup.