Casillas debuted for Spain at the under-17 level. At age 16, he was the youngest player in the Spanish squad that placed third at the 1997 FIFA U-17 World Championship in Egypt. He was later made captain of the U17s. Two years later, he went on to win the FIFA World Youth Championship and the UEFA–CAF Meridian Cup that same year. Initially second-choice, he soon worked his way up to first-choice and eventually earned his first senior cap following some brilliant performances at club level.