In this work, photocatalytic oxidation of As(III) to As(V) by ZnO photocatalyts doped with Cu was investigated under visible light. Photocatalysts were successfully prepared by precipitation method. The obtained samples were characterized by N-2 adsorption at -196 C, X-Ray fluorescence analysis, X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV-vis reflectance analysis. In particular, according to XRD analysis, all samples showed an hexagonal wurtzite structure with average crystallite sizes approximately in the range 32-33 nm. The doping with Cu allowed to obtain a decrease of band gap energy from 3.2 eV, typical of pure ZnO, to 2.92 eV. Photocatalytic oxidation tests under visible light showed that undoped ZnO is not able to oxidize the As(III) present in solution, while the complete conversion of As(III) to As(V) was achieved in the presence of Cu doped ZnO photocatalyst. The best photocatalytic activity was observed with ZnO doped with 1.08 mol% of Cu (1.08Cu_ZnO), within 120 min of exposure to visible light irradiation. The same result was observed under solar simulated radiation. Photocatalytic tests were carried out also in real drinking water and complete oxidation of As(III) to As(V) by 1.08Cu_ZnO photocatalyst was achieved in 120 min under both visible light (emitted by LEDs) and solar simulated radiation.