A plasmon-enhanced water splitting system using a gold nanostructured strontium titanate (SrTiO3) single-crystal photoelectrode with a titanium dioxide (TiO2) thin film heterojunction was developed. Simultaneous evolution of hydrogen and oxygen was observed in separate reaction chambers under visible light irradiation. The substrate composed of single-crystal SrTiO3 with a rutile TiO2 thin film heterojunction exhibited enhanced water splitting activity compared with the electrode without the TiO2 thin film because the synergistic effect may prevent the back-electron-transfer reaction.