The agency said it would support Co-op Food, which claims on its website to be the UK's fifth biggest food retailer with almost 2,800 stores nationally, with a series of creative "big and bold" campaigns alongside "creative news generation".
PRWeek understands this is the first time Co-Op Food has had its own retained agency and there was no incumbent.
M&C Saatchi PR’s brief is help reposition the retailer and amplify how it is different to competitors, after the firm announced last month it was going back to "being Co-op" and returning to its original logo.
The agency said it would work alongside an unnamed digital agency on the Co-op account.
Activity will begin with the crucial pre-Christmas trading period and the agency is expected to work with Co-op through 2017.
Kat McGettigan, head of consumer at M&C Saatchi PR, said: "We are absolutely thrilled to welcome Co-op to the M&C Saatchi PR family. We have identified a list of brands we want to work with and are passionate about. The Co-op is one of those brands and it’s an exciting time to come on board as the Co-op goes back to its founding principles and purpose."
Co-op’s head of retail PR Craig Noonan said the agency had won in a "tough pitch process" because it understood the supermarket’s brand values, as well as the retail marketplace.
Other clients of the agency have included EE, Paddy Power and Ocado.