The Catalan giants confirmed on Wednesday evening that De Jong will join club from next season in a deal worth 75 million plus 11 million in add-ons.
De Jong, who is already a Netherlands international, has signed a five-year deal with LaLiga champions.
A statement from Barca read, "FC Barcelona and Ajax have reached an agreement for the transfer of midfielder Frenkie de Jong, who will be joining the Catalan club from 1 July 2019.
The cost of the transfer operation is 75 million euros, plus a further 11 million in variables. The player will be signing a contract for the next five seasons, through to 2023/24.
The president Josep Maria Bartomeu and the CEO Òscar Grau were personally involved in closing the deal in Amsterdam.
De Jong becomes Barca's signing for the 2019/2020 season after they secured the services of young French defender Jean-Clair Todibo from Toulouse.