The agency will work across the trade body's direct marketing, advertising, website, events and awards, and on supporting marketing materials.
Lindsey Clay, the director of marketing and communications at Thinkbox, said: "The Red Brick Road showed the deepest strategic understanding of the issues surrounding commercial TV as well as the opportunities the new TV landscape offers advertisers. It had a clear vision for how it can help Thinkbox to deliver its ambitions."
Thinkbox launched in February 2005 and is backed by the main UK commercial broadcasters: Channel 4, five, GMTV, ITV, Sky Media, Turner Broadcasting and Viacom Brand Solutions.
Paul Hammersley, the managing partner of The Red Brick Road, said: "We pitch for the things we care about. Nothing could be more important than the future of TV and Thinkbox's task."