The agency was appointed following a four-way competitive pitch and will handle the comms around the events, which attract 170,000 people between them.
It will work with print and online, including national, specialist, regional and student titles.
Key messaging will include a focus on the added value of next year's shows, following the addition of extra stages.
Jamie Stockwood, founding director of The Zeitgeist Agency, called the win ‘the pinnacle of festival work’ for his agency.
He said: ‘The events are well liked by media and the messages about the diversity of what's on offer onsite will be well articulated. In addition to the key supporters in the music and festival-focused areas of the media, we will broaden the Reading and Leeds story out into more men's, women's, specialist music, gaming and action sports titles.’
The two festivals will take place over the course of a weekend next August, with acts for the 2012 festivals including Foo Fighters and Florence and the Machine.
Melvin Benn, MD of Festival Republic, added that the agency’s ideas around reaching new areas of the media were a major factor in the decision.
‘I feel they know the festival, the culture of the festival and the festival fans,’ he added.