On 19 October 2002, de Jong played for the Ajax first team for the first time. He scored his first goal on 18 February 2003 in a 1–1 draw against Arsenal in the UEFA Champions League. He started playing in the first team often in the next season. In this season, Ajax won the Eredivisie. In 2004–05, de Jong was named Ajax Player of the Season. Later, after starting in midfield for quite a while, he found himself sitting on the bench more often than being one of the players at the start of a match. On 7 December 2005, it was announced that de Jong did not want to stay with Ajax. His contract with Ajax was due to finish in July 2006.