The Netherlands global arrives at Selhurst Park in a reported £7.9million move having impressed with the Eredivisie side last season.
Having got the deal over the line on Monday evening, De Boer is looking forward to once again working with the Netherlands worldwide. In doing this, he broke the respective records for the youngest club scorer and the youngest Eredivisie scorer at just 17 years and 104 days.
Capable of playing at centre-back, left-back as well as in defensive midfield, Riedewald can develop into an important squad member at Selhurst Park and fans will hope he can help fill the void that Mamadou Sakho has created since returning to Liverpool following a successful loan spell.
Last season, Riedewald made seven appearances during Ajax's run to the Europa League final, where they suffered a 2-0 defeat to Manchester United. The Haarlem-born defender has gone on to win a further two caps for his country and also featured at every youth level from under-15 to under-21.
"I look forward to meeting my new team mates and then getting the chance to be out on the training ground and getting ready for the new season".