Novel cysteic acid/graphene oxide composite film on glassy carbon electrode has been developed through the electrochemical oxidation of L-cysteine (CySH) for constructing an all-solid-state pH sensor. Potentiometric pH response of the cysteic acid/graphene oxide modified electrode in Britton–Robinson buffers revealed a linear working range from pH 2 to12 with a slope of about −50.2 mV·pH⁻¹ and fast response (less than 1 minute). The as-developed pH sensor also showed excellent reproducibility and high stability and had been successfully used to detect real samples with satisfactory results.