Leila Deen (born 1979) is a British environmental activist, campaigning on the issues of climate change, poverty and water politics. She is program director at SumOfUs in Washington, DC. Previously, she led Greenpeace UK's campaign against fracking and was projects director with Greenpeace USA until 2019. She was previously an activist with the World Development Movement and Plane Stupid. She is most widely known for pouring green custard on the then Business Secretary Lord Mandelson in March 2009 in protest against the extension of Heathrow Airport, for which she was arrested and cautioned.