Plasmonic-metal nanostructures exhibit broadly tunable optical properties coupled with catalytically active surfaces that offer unique opportunities for solar photocatalysis. Of particular interest is the resonant optical excitation of surface plasmons to produce energetic “hot” carriers at both metal-semiconductor and metal-electrolyte interfaces that can drive photochemical reactions. While examples of hot-electron-driven carrier collection and photoelectrochemical processes have been widely reported, little is known about the nature of plasmon-derived hot holes and their role in hot carrier photocatalysis. Here, we report the demonstration of a plasmon-driven photoelectrochemical CO2 reduction process occurring in a Au/p-type GaN photocathode. Hot-hole collection is observed to occur by hole injection from gold (Au) nanoparticles into a p-type gallium nitride (p-GaN) semiconductor support. Despite an interfacial Schottky barrier to hole transport of more than 1 eV across the Au-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes into GaN upon plasmon excitation of Au nanoparticles. The photocurrent spectral response of our plasmonic Au/p-GaN photocathode faithfully follows the surface plasmon resonant absorption spectrum of the Au nanoparticles and open-circuit voltage studies demonstrate the ability to sustain a plasmonic photovoltage of 20 mV across the Au-GaN heterojunction during plasmon excitation. For comparison, Au/p-NiO heterojunction were formed by Au nanoparticle deposition onto p-type nickel oxide (p-NiO) photocathodes, for which there is no Schottky barrier across the metal-semiconductor heterojunction. Significantly, there is little difference in device performance between these two distinct systems, supporting previous theoretical predictions about the distribution of hot holes deep below the Au Fermi level. The plasmonic Au/p-GaN photocathodes were further employed for plasmon-driven CO2 reduction in aqueous electrolyte, illustrating the concept of plasmon-driven artificial photosynthesis. Taken together, our results offer experimental verification of optically-excited hot holes more than 1 eV below the Au Fermi level and demonstrate a photoelectrochemical platform for harvesting them to drive solar-to-fuel energy conversion.