One of the most important challenges in renewable energy conversion is construction of an earth-abundant, active, and stable water splitting photoelectrocatalyst operating at all pH values. In this work, ZnS nanoparticles (NPs) were embedded on different amounts of reduced graphene oxide (RGO) nanosheets with the help of ultrasonic waves. The role of the RGO-ZnS nanocomposites as an efficient photoanode for oxygen evolution reaction was evaluated. The RGO-ZnS nanocomposite with 10% RGO (RGO10-ZnS) exhibits outstanding photoelectrocatalytic activity compared with ZnS, RGO, and other prepared nanocomposites. It shows a negative onset potential of -200 mV with a maximum photocurrent density of 1.1 mA cm(-2) at 1.23 V (vs RHE) in Na2SO4 (0.5 M) solution which is five times higher than that of ZnS NPs. The solar-simulated light power is saved 1 V from the external power source or produced power of 0.9 mW cm(-2). RG010-ZnS showed -400 mV increase in the photovoltage than that of ZnS nanoparticles. The excellent conductivity and high surface area of RGO provide an outstanding ability to the nanocomposite to harvest and localize the light near ZnS NPs through its plasmonic structure which causes a negative shift in the onset potential compared with both ZnS and RGO. In other words, RGO has the ability to trap light in modes so-called surface plasmons. The light surface plasmon resonances can efficiently enhance the optical absorption at resonant frequency to slow down the photons.