Chris Bain is to retire in December after 15 years as director of Cafod, the Catholic aid agency announced today.
The charity, which employs about 400 staff and had an income of £50m in the year ending 31 March 2017, said in a statement it would begin the search for a successor "in the coming months".
Bain, 64, has had a long career in international development dating back 40 years, beginning as a VSO volunteer.
At Oxfam, he was part of the Free Front Line Africa campaign, which called for sanctions against the apartheid regime in South Africa.
He became VSO's head of international programmes in 1999 and began his current role at Cafod in 2003.
Under his leadership Cafod has been at the forefront of action on climate change, successfully lobbying the government to bring in the Climate Change Act 2008, which set legally binding carbon targets, the charity said.
It has also led the humanitarian response to crises such as the 2015 Nepal earthquake, which killed 9,000 people.
Bain said in a statement: "Cafod's sense of mission and purpose has never been stronger. We are in sound financial health and have a wealth of talent in our dedicated staff and volunteers."
Bishop John Arnold, chair of Cafod, said: "Chris has led Cafod through great successes in challenging times.
"He leaves Cafod having provided a clear directional purpose and strong sense of mission."