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Strong ultraviolet luminescence having intensity comparable with device-quality GaN epifilms has been observed in Au nanoparticles. It is identified that the luminescence involves radiative recombination of electrons in band 6 (sp conduction band) with holes in band 4 (secondary top d band), near the L symmetry point. We show that the strong emission is a consequence of the second-harmonic surface plasmon resonance (SHSPR), which is an inherent nature of metallic nanoparticles with high density of surface plasmons. The newly discovered SHSPR is very different from the conventional second-harmonic generation (SHG). In the conventional SHG, it requires an intense incident laser to generate two-photon absorption and radiation. However, for the SHSPR discussed here, we only need a weak pumping source to trigger the second-harmonic absorption of very dense surface plasmons in Au nanoclusters. In addition to Au nanoparticles, we demonstrate that SHSPR provides a very efficient way to enhance the luminescence of a material incorporated with metal nanoparticles. As an example, incorporation of Au nanoparticles into SiO2 nanoparticles can enhance the luminescence intensity by two orders of magnitude. We thus point out that SHSPR can serve as one of the underlying mechanisms responsible for surface-enhanced nonlinear optical phenomena.