The 21-year-old will join Barcelona in July for a fee of 75m Euros, plus a further 11m in possible add-ons.
Barcelona beat Premier League champions Manchester City and Paris Saint-Germain to the signature of the highly-rated teenager.
De Jong has signed a five-year deal and becomes the latest highly-rated Dutch player to join Barcelona, a tradition that includes Johan Cruyff, Ronald Koeman, Patrick Kluivert, Frank de Boer and current goalkeeper Jasper Cillessen.
Barcelona club president Josep Maria Bartomeu said De Jong is joining an exciting project. “With the arrival of Frenkie De Jong we add talent, youth and Barça style to our sporting project. We are convinced that he will be a key part of our team for years to come. Welcome, Frenkie!”
The youngster also has the seal of approval from former club captain Xavi. "Securing a talent like Frenkie de Jong seems like a return to their first principles of passing and possession," said Xavi.
De Jong signed for Ajax after impressing at the Willem II academy and signed a four-year contract with the Amsterdam club in 2015. He has since made five international appearances as part of Koeman's Netherlands side after making his debut in June 2019.
De Jong becomes Barcelona's second major signing of the January transfer window after Kevin-Prince Boateng joined the Catalan club on Tuesday. "It's a big dream come true," he said. "It's a big honour for me to be here."
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