The former Chippa United and SuperSport United player, who can feature in defence or midfield, had been training with the Urban Warriors for the past two weeks.
The 27-year-old finally did enough to convince coach Roger de Sa that he deserves a return to the top-flight after spending time on loan from SuperSport with Vasco da Gama last season.
Pietersen says he is ecstatic to further his career at Ajax.
"I'm confident I can add value to Ajax," he told snl24.com/kickoff. "We have assembled a competent squad with a good mix of experience and young players.”