Via VK Tielrode and Germinal Beerschot Vertonghen moved to the Netherlands and signed a contract with Eredivisie side Ajax. In a 6-0 win over Willem II in the Amsterdam ArenA Vertonghen made his professional debut on December 3, 2006. He would play another two matches before the winter break and during this break he was sent on loan to struggling RKC Waalwijk for the remainder of the season. At RKC Waalwijk he would play 12 matches in which he scored three times, but was unable to help them avoid relegation to the Eerste Divisie.
On June 2, 2007 he made his debut in the national team in which they lost 2-1 to Portugal. Vertonghen returned to Ajax for the 2007-08 season, he is tip for stardom, but has not broken through just yet, due to a bad injury, but Johan Cruyff has made no secret of his admiration for the youngster who can handle any left-sided position in defence and midfield. On 26 September 2008 he extended his contract to 30 June 2013.