The agency scooped the business after a final pitch against the WPP-owned Maxus. PHD, the incumbent, declined to repitch.
The Publicis-owned agency will now work on BBC Worldwide brands such as Top Gear, BBC America, Planet Earth and Doctor Who.
BBC Worldwide, the commercial arm of the BBC, said the move was part of its strategy to reduce its volume of suppliers and develop high-quality marketing campaigns to drive significant growth outside the UK.
Helen Kellie, the global director of marketing for BBC Worldwide, said: "We are delighted to appoint ZenithOptimedia as our new global agency. Through this partnership we aim to focus on building greater awareness, presence and revenues for the BBC."
PHD first took on BBC Worldwide's media account in December 1998.