The agency won the business after a pitch against Addiction London, Albion and Quiet Storm, and will be responsible for creating a new brand idea that can work across a number of channels and be used for sales activity.
Comet called a pitch for its advertising business in October last year, in reaction to increased marketing activity from competitors such as PC World and Argos.
Saatchi & Saatchi, the incumbent on the account since 1995, declined to repitch for the business.
Comet's recent advertising campaigns have predominantly focused on price. The radio presenter Edith Bowman has provided the voiceover for the ads.
The electronics retailer recently announced a fall in sales of 3.9% during the Christmas period.