The agency was appointed after a five-way pitch and will develop a campaign to target children, particularly those aged 11-13, with messages about healthy eating.
The heart charity's most recent campaign, created by Bright Blue Day, picked up a silver at the 2007 Campaign Poster Awards for its image of a girl drinking from a bottle of cooking oil.
Betty McBride, the British Heart Foundation's director of policy and communications, said: "The online world feels like a Wild West environment for junk-food marketers. We're going to use that battleground ourselves now and deploy sophisticated techniques to get our message across to help children make informed choices."
A government report issued last month estimated that around half the population could be obese by 2050.