The Amsterdam giant sacked Alfred Schreuder a week ago following a run of no wins in seven Eredivisie matches, its joint-worst winless streak in the league's history.
Schreuder had only been in charge since the start of the season, when he was appointed as Erik ten Hag's replacement following his departure to Manchester United.
Ajax's lack of victories saw them drop to fifth in the table, but they bounced back on Monday (AEDT) with a 4-1 win at Excelsior.
Heitinga – who had been in charge of the Under-23s – was at the helm for the trip to Rotterdam, and will now take the senior side through to the end of the campaign.
The former Ajax, Atletico Madrid and Everton player, who played internationally for the Netherlands, has held various youth coaching positions at the club since retiring as a player in 2016.
His first match as head coach will be away to second-from-bottom Cambuur on Sunday.
Ajax is fourth in the Eredivisie but only trails leader Feyenoord by five points.