Cu doped ZnS nanoparticles (Zn1−xCuxS; where x=0.00, 0.03, 0.05 and 0.10) were synthesized by a chemical precipitation method. The synthesized products were characterized by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope, ultraviolet-visible and photoluminescence spectrometer. The X-ray diffraction and high resolution transmission electron microscope studies show that the size of crystallites is in the range of 2–10 nm. XRD study revealed that the samples are composed of cubic phase without doping and at 3 mol% Cu doping concentration while at the doping of 5 mol% and 10 mol% Cu, phase transition from cubic blende to hexagonal phase occurs in ZnS. Photocatalytic activities of ZnS and 3, 5 and 10 mol% Cu doped ZnS were evaluated by decolorization of methylene blue in aqueous solution under ultraviolet and visible light irradiation. It was found that the Cu doped ZnS bleaches methylene blue much faster than the undoped ZnS upon its exposure to the visible light as compared to the ultraviolet light. The optimal Cu/Zn ratio was observed to be 3 mol% for photocatalytic applications