Rapp Media has also been retained to handle Somerfield's media business. Rocket will handle the bulk of the Co-op's food and banking business.
The supermarket chain held a review of its media account in April after its £1.6m acquisition of Somerfield in February.
The businesses in the review included Co-op Travel, Co-op Insurance Society, Co-op Bank, Co-op Funeralcare, Priority Motor Group, Unity Trust Bank and Co-op Financial Services, which counts internet bank Smile among its brands.
Somerfield spent £12m on media in 2008, while the Co-operative Group spent a total of £42.4m, according to The Nielsen Company.