Hierarchical MoS2-reduced graphene oxide (RGO) nanocomposites were successfully synthesized via a hydrothermal synthesis approach, and their morphology, crystal structure of the composites as well as the photocatalytic activities in the degradation of methylene blue (MB) were investigated using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and UV–vis absorption spectroscopy. The results demonstrated that the MoS2-RGO nanocomposites exhibited excellent photocatalytic performance with a maximum degradation rate up to 80% under visible light irradiation for 30 min. To understand the origin of the excellent photocatalytic properties, first-principles calculations were also carried out. It manifested that the excellent photocatalytic activity should be resulted from the existence of the potential electron transfer between conduction band maximum (CBM) of MoS2 and CBM of RGO. Meanwhile, the enhanced visible light absorption, reduced electron–hole pair recombination, and enhanced surface area for absorption of dyes also could be the factors for the excellent photocatalytic performance.