The agency will publicise the End Hunger: Walk the World event, a sponsored 5km walk for the charity. The event takes place on 1 June in more than 90 countries. Six Degrees will handle media for the UK leg in Windsor.
The agency will target regional media to encourage families and children to take part in the event. It is concentrating on broadcast, radio and local press.
'This is the world's largest annual global fundraising event,' said director Amanda Hassall.
Six Degrees has worked with the WFP before on a pro-bono basis, but TNT Post is paying the agency to handle the media work.
The WFP is the food aid branch of the UN. TNT provides airlift services in emergencies and specialist support to the organisation.