Curtis, a historian and fierce critic of British foreign policy, said he wanted to return to writing. He will leave the organisation in May.
He said his decision did not stem from any disagreement with the international campaigning group, but was "a personal choice about where to put my energy and attention". But he admitted feeling "a bit uneasy" about leaving less than a year after his appointment.
After leaving Christian Aid in 2003, Curtis criticised aid agencies for "hob-nobbing with ministers" and becoming co-opted by the powerful. He replaced Barry Coates as director of WDM last summer.
- See News, p4.