OgilvyOne won the charity's account in April 2010, beating off competition from two undisclosed agencies.
It created a campaign that launched in December and included a TV ad that appealed to viewers to sponsor a child in a third-world country, and ensure they have access to clean water.
OgilvyOne said that during its one-month notice period, it would co-operate with ActionAid UK and any successor agencies to ensure a smooth transition on the account.
At the end of November, ActionAid International ran an in-house created press ad in UK newspapers, attacking SAB Miller, the owner of beer brand Grolsch, on its tax record in Africa.
SAB Miller is a client of Ogilvy SA, the South African communications group in which WPP's Ogilvy & Mather recently announced it was to increase its stake from 49.9% to 59%.
An OgilvyOne spokesman denied there was a connection between ActionAid's SAB Miller ads and the agency resigning the account.
ActionAid did not respond to requests for comment.