The 23-year-old made 71 appearances in two seasons at Heracles, scoring 10 goals, and made a fine start to the Eredivisie campaign, netting twice in five games.
Duarte's arrival will offset the departure of Christian Eriksen, who has joined Tottenham, and the former Netherlands Under-21 international has taken the number eight shirt previously worn by the Dane.
After working his way up through the ranks at Sparta Rotterdam, Duarte made his professional debut against Twente in August 2009 and went on to make 50 appearances, scoring seven times.
He has made 10 appearances for Netherlands Under-21s, also notching one goal for the national youth side.
Duarte is Ajax's third signing of the season, following the arrivals of former Utrecht defender Mike van der Hoorn and Bojan Krkic from Barcelona on loan.
