The home-improvement retailer, which has a marcoms budget of more than £50 million, will start working with WCRS in February and aims to increase customer engagement.
Karmarama had worked on the account since November 2012, when it beat WCRS in the final stage of a pitch.
B&Q and Karmarama split last month after a series of management changes at the retailer, including the arrival of the customer and marketing director, Chris Moss.
The company has also hired the PR agency Good Relations to handle all brand and product communications. B&Q’s media agency, MEC, was unaffected.