Zinc oxide (ZnO) nanoparticles were synthesised from zinc acetate as a precursor and PVP as a capping agent by polyol method. The as prepared ZnO nanoparticles were doped with 0.01 M solution of copper acetate in PVP. Both ZnO and Cu-doped ZnO nanoparticles were characterized by UV-Vis spectroscopy, X-rays diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and energy dispersion X-rays (EDX) spectroscopy. The sunlight irradiated photocatalytic degradation of Reactive Red S3B was carried out using both ZnO and Cu doped ZnO nanoparticles. Both ZnO and Cu-doped ZnO decolourize the respective dyes but Cu-doped ZnO nanoparticles were clearly more powerful catalyst than ZnO. The kinetics of the photocatalytic degradation was also studied. It was found that the degradation of Reactive Red S3B followed the pseudo first-order kinetics. These photocatalysts are efficient and environment-friendly and can be highly useful for the wastewater treatment contaminated with these carcinogenic synthetic dyes.