In this article, we report the enhancement of the photocatalytic activity of ZnO/carbon dots (CDs) nanocomposite in degrading methylene blue (MB) dye. Carbon dots (CDs) were synthesized by a hydrothermal method utilizing coffee grounds as a carbon source. ZnO/CDs nanocomposite was synthesized by a single-step and in-situ sonochemical process. The particle size of CDs measured by dynamic light scattering (DLS) was found at about 7.7 nm. The result of X-ray diffraction analysis confirmed that the hexagonal wurtzite structure of ZnO nanoparticle has a small crystal size and the lattice parameters decreasing due to CDs addition. The surface morphology images were taken by scanning electron microscopy (SEM) shows a very fine particle both for pure ZnO nanoparticle and ZnO/CDs nanocomposite. The result of Energy Dispersive X-Ray Spectroscopy (EDXS) analysis confirmed the presence of the carbon element in the ZnO/CDs nanocomposite indicated that the nanocomposite has been formed. The result of the photocatalytic test showed that the ZnO/CDs nanocomposite photocatalyst had significantly increased photocatalytic activity compared to the pure ZnO nanoparticle photocatalyst. ZnO/CDs nanocomposite photocatalyst resulted in a photodegradation rate three times higher than pure ZnO nanoparticle photocatalyst and nine times higher than the photolysis treatment (without photocatalyst). Whereas the degradation efficiency of MB using ZnO/CDs nanocomposite photocatalyst was twice higher than using pure ZnO nanoparticles and four times higher than photolysis treatment.