Notices have been put up in the Costcutter supermarket at the RÃ¡th MÃ³r shopping centre in Creggan.
Management said it would: "Reserve the right to refuse admission to anyone wearing bed-time clothing i.e. pyjamas, onesies in this store."
Shop staff said the sign had been up "for some time". The BBC have asked Costcutters for a statement.
Earlier this year, Tesco 'shrugged off' a complaint from a customer to ban shoppers wearing nightclothes in its stores.
A Londonderry shop has asked customers not to shop in their pyjamas or onesies.