A novel form of polyvinylpyrrolidone (PVP) coated copper(I) oxide nanoparticle (Cu2O-NP) was prepared and used to catalyze azide-alkyne click reactions in water under aerobic conditions. The nanoparticles were well dispersed in aqueous solutions and have a size of 20±10 nm, as determined by transmission electron microscope (TEM). Inductively coupled plasma (ICP), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses demonstrated that the main content of Cu2O-NP is copper(I). The cytotoxicity of it was evaluated by an in vitro 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and its catalytic efficiency for azide-alkyne click reactions was studied in water and organic solvents at physiological temperatures. Our results indicate that Cu2O-NP is more efficient in catalytic reactions in water for both aliphatic and aromatic azides and alkynes and less toxic than the commonly used CuSO4/reductant catalyst systems.