Light absorption at the surface of a photodiode can be enhanced by employing nanostructures smaller than the wavelength of interest. In this study, a ZnO quantum dot (QD) coating layer was investigated for improving the light absorption of gallium nitride (GaN) ultraviolet (UV) photodiodes. A GaN surface coated with a ZnO QD solution exhibited significantly lower surface reflection than an uncoated GaN surface, which, in turn, improved the responsivity of the GaN photodiode. In comparison with other nanostructure or multilayer thin film processes, the proposed ZnO QD coating process is simple and effective in enhancing UV light absorption.