Ajax said in a statement it had signed Greece's Euro 2004 hero until June 30, 2008. No financial details were given.
German media reported on Thursday that Bremen wanted five million euros ($6.72 million) for Haristeas, 24, who joined the Bundesliga side from Aris Saloniki in 2002.
Haristeas won the double with Werder last season and helped Greece to victory at Euro 2004, scoring three goals including the winner in the 1-0 victory over Portugal in the final.
Ajax were knocked out of the Champions League earlier this month after finishing third in Group C but the 1995 European champions scrambled to a UEFA Cup berth.
Two years ago, Ajax reached the Champions League quarter-finals with one of their youngest teams ever but management failed to replace such players as Christian Chivu, Andy van der Meijde and Zlatan Ibrahimovic.
