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The ability to convert low-energy quanta into a quantum of higher energy is of great interest for a variety of applications, including bioimaging 1 , drug delivery 2 and photovoltaics 3 . Although high conversion efficiencies can be achieved using macroscopic nonlinear crystals, upconverting light at the nanometre scale remains challenging because the subwavelength scale of materials prevents the exploitation of phase-matching processes 4 . Light–plasmon interactions that occur in nanostructured noble metals have offered alternative opportunities for nonlinear upconversion of infrared light, but conversion efficiency rates remain extremely low due to the weak penetration of the exciting fields into the metal 5 . Here, we show that third-harmonic generation from an individual semiconductor indium tin oxide nanoparticle is significantly enhanced when coupled within a plasmonic gold dimer. The plasmonic dimer acts as a receiving optical antenna 6 , confining the incident far-field radiation into a near field localized at its gap; the indium tin oxide nanoparticle located at the plasmonic dimer gap acts as a localized nonlinear transmitter upconverting three incident photons at frequency ω into a photon at frequency 3 ω . This hybrid nanodevice provides third-harmonic-generation enhancements of up to 10 6 -fold compared with an isolated indium tin oxide nanoparticle, with an effective third-order susceptibility up to 3.5 × 10 3  nm 2  V −2 and conversion efficiency of 0.0007%. We also show that the upconverted third-harmonic emission can be exploited to probe the near-field intensity at the plasmonic dimer gap. The third-harmonic-generation efficiency of an individual indium tin oxide nanoparticle is enhanced by more than 10 6 fold by placing it within the gap of a plasmonic gold dimer nanoantenna.