Andy Atkins is currently policy and campaigns director of the relief and development charity.
He will join Friends of the Earth in June, before replacing current executive director Tony Juniper later in the summer.
Atkins has been at Tearfund for eleven years, holding his most recent position for the last eight.
He was a key organiser of the Make Poverty History campaign and is a board member of the Stop Climate Chaos coalition, where he works with Juniper.
Juniper announced he was standing down from his position earlier this year.