De Ligt, who had also been linked with Manchester United, Barcelona and Paris-St Germain, has signed a five-year contract with the Serie A side.
Juventus said the fee was “payable in five financial years” with “additional costs of 10.5m euros (£9m)”.
De Ligt helped Ajax to the Champions League semi-finals last season.
He made his debut for the Amsterdam club in 2016 and became their youngest captain in March 2018.
De Ligt scored eight goals in 77 league appearances, winning the Dutch domestic league and cup double last season, as well as reaching a Europa League final in 2017.