Nano-ZnO supported on bentonite was prepared to form composite photocatalyst by sol-gel method. The photocatalyst was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM). C.I. Acid Red 35 was used as simulating contaminant to be treated by ultraviolet light synergistic with nano-ZnO/bentonite. The results show that 5.7 nm ZnO particle was acquired and uniformly dispersed on the surface of the bentonite at calcination temperature of 200°C. The removal of C.I. Acid Red 35 could reach 84.9% after 200 min under optimum ZnO/bentonite dosage of 0.6 g L−1. The 60% ZnO content in ZnO/bentonite composite exhibited a great photocatalytic activity to treat C.I. Acid Red 35. The photocatalytic process followed pseudo-first-order kinetics and the best apparent rate constant was 0.00927 min−1 with correlation coefficient (R2) of above 0.98.