In this study, an electrochemical sensor for the detection of herbicide diuron in natural water using a reduced graphene oxide-gold nanoparticle (rGO-AuNP)-modified screen-printed electrode (SPE) was developed. The proposed sensor was fabricated by the simple electrochemical co-reduction of graphene oxide and chloroauric acid using a cyclic voltammetry (CV) technique and applied to the direct electrochemical detection of diuron residue in water without using any mediator. A field emission scanning electron microscope image showed that the AuNPs were firmly attached and well distributed on the surface of the rGO nanosheets. The presence of rGO nanosheets was further proven by the Raman spectrum, which reveals that the D/G intensity ratio for rGO was smaller than that for GO. Moreover, the CV and linear sweep voltammetry results showed the effective accumulation of diuron on the rGO-AuNP/SPE surface and produced an irreversible reduction peak at around -0.5 V in a pH 5.5 phosphate buffer. Hence, it greatly enhanced the electrochemical reduction of diuron. Under optimized conditions, the cathodic peak current was proportional to the diuron concentration over a wide range of 0.5-30.0 A mu g mL(-1), with a detection limit of 0.125 A mu g mL(-1) (S/N = 3). The proposed diuron electrochemical sensor also exhibited a relative standard deviation of 4.25 % for a six replicate analysis of 10.0 A mu g mL(-1) diuron, and the response of the electrode declined by 20 % after 30 days at ambient temperature. In addition, the sensor was successfully employed for the determination of diuron in a variety of water samples such as lake and seawater sample.