Green Alliance director Stephen Hale is leaving to take up the post of deputy advocacy and campaigns director at Oxfam International in Geneva on 30 April.
In a statement, the Green Alliance said that Hale had four successful years as director of the Green Alliance and that the organisation had grown dramatically in size, reputation and influence during his tenure as director.
Hale said: The past four years have flown by. I am immensely proud of Green Alliance's achievements during that time and our success in collaborating with organisations across the voluntary sector to exert influence on our political leaders. I know the organisation will continue to go from strength to strength and look forward to watching this from my new vantage point at Oxfam International.
The Green Alliance is due to announce a successor to Hale next week.