Herein, we report the fabrication of RGO-ZnS nanocomposites by a facile ultrasound assisted hydrothermal process. Three nanocomposites with different weight ratios of RGO namely RGO3-ZnS, RGO5-ZnS and RGO10-ZnS were prepared and characterized by several techniques. The photocatalytic activity of ZnS, the nano composites, and P25 was evaluated against Reactive black 5 (RB5) under sunlight irradiation. A high rate constant of 3.05 x 10(-2) min(-1), the chemical oxygen demand (COD) value of about 10%, and the mineralization efficiency of 90% were obtained within 300 min when RGO3-ZnS was used as a photocatalyst. To elucidate the photocatalytic mechanism of RGO3-ZnS nanocomposite, the reactive oxygen species trapping experiments, photoluminescence, linear sweep voltammetry, and open circuit potential decay measurements were performed. On the basis of the experimental results, the enhanced photocatalytic activities of RGO3-ZnS could be ascribed to the increased dye adsorption, optical absorption and charge carrier generation, efficient separation, and migration of photogenerated charge carriers as well as the decreased recombination probability of electron-hole pairs as a result of introducing of RGO in the nanostructure.