Jam won the business following a four-way pitch with undisclosed agencies.
The move marks Comet's first foray into social media marketing and Jam's brief involves reputation management and buzz research.
Comet is currently undergoing a review of its £13m advertising business.
The incumbent, Saatchi & Saatchi, decided not to repitch in November due to client conflicts arising from its appointment to Asda's advertising business in the same month.