Barry Coates, director of NGO the World Development Movement, is to leave the charity at the end of September.
He will return to his native New Zealand to become director of Oxfam in that country.
Coates has been with the WDM for seven years, during which time the organisation has doubled its income and expanded staff by one-third.
Coates said he was he was proud of the "strong campaigning" that WDM had undertaken on issues such as untying aid from trade, the Jubilee debt campaign and the campaign against the Multilateral Agreement on Investment.
He also said that the WDM had built coalitions with other NGOs outside the development field to form a "wider civil society movement".
In 2001, a leaked document revealed that a joint committee of senior civil servants and representatives of the financial services industry were discussing how to counter WDM campaigns against aspects of the General Agreement on Tariffs and Trade.