There is no incumbent on the business, which has been previously handled by Hangar Seven. Prior to that, M&C Saatchi had picked up the account in 2010, when it was worth £6 million.
Claire Bayliss, the chief marketing officer at Bathstore, said: "Having spent a considerable amount of time in developing our brand strategy, we wanted to take the next step and bring on board an agency that could help us tell the Bathstore story and support our continued growth."
Saatchi & Saatchi will create a brand campaign that will launch later in the year, building on Bathstore’s "service by design" tag line.
The activity will run in film, digital, social and retail channels.
Magnus Djaba, the chief executive at Saatchi & Saatchi Fallon UK, said: "Bathstore is an entrepreneurial brand and a growth business with a spirit that matches our own."