Graphical abstract: CuS-reduced graphene oxide nanocomposite has excellent photocatalytic degradation efficiency towards Congo Red degradation under sunlight irradiation due to synergistic effect between CuS and reduced graphene oxide sheets. Semiconductor based CuS-rGO nanocomposite materials have drawn a considerable attention towards photodegradation of organic dye molecules due to the low band gap (∼2.5eV) of CuS nanoparticles. Presence of reduced graphene oxide (rGO) in CuS-rGO nanocomposite induced synergistic effect between CuS and rGO sheets which led towards better photocatalytic degradation efficiency as compared to CuS nanoparticles (without rGO support) and rGO alone. In this study CuS-rGO nanocomposite was synthesized by a simple microwave irradiation technique and characterized by High resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and Photoluminescence (PL) spectroscopy studies. The synthesized nanocomposite behaved as efficient photocatalyst towards diazo Congo Red (CR) dye molecule under natural sunlight irradiation with a maximum degradation efficiency of 98.76%. Effect of initial dye concentration, catalyst loading, pH of the reaction medium and role of different inorganic ions as well as the amount of graphene content in the photocatalyst on photocatalytic degradation of CR dye molecule was investigated in this study. The present study also focussed on the effect of different inorganic ions on the surface potential of the photocatalyst and their effect on the degradation process.