It will be briefed to create a campaign focusing on the indulgence and health benefits of dark chocolate.
The advertising, which will break next year, will focus on Bournville chocolate's heritage. It is expected to include marketing for a new, higher cocoa content version of the 98-year-old brand.
The Fallon founding partner Laurence Green said: "It's a privilege and responsibility to help relaunch a brand such as Bournville."
This appointment sees Fallon join a roster that includes Publicis, which handles Cadbury-branded lines, and Euro RSCG London, which holds the company's Trebor and Maynards brands. Neither agency is understood to have been approached in relation to the Bournville task.
The relaunch comes as Cadbury moves to market its Green & Black's organic dark chocolate brand more aggressively in the lead-up to Easter. The Green & Black's above-the-line account is currently held by Brave.
Last month, Mark Palmer announced he was to stand down as the marketing director of Green & Black's. He will remain a director of the company. His role has been filled by Carol Welch, who moves across from Cadbury.