AbstractMuch attention has been paid to the development of high‐performance electrocatalytic reactions by exploiting the plasmonic properties of noble metal nanoparticles. In this study, ultrathin Pt layers with different coverage ratios were prepared on plasmonic octahedral Au nanoparticles (NPs) by underpotential deposition (UPD) of Cu followed by redox replacement with Pt. The effect of the Cu UPD cycle on the deposited Cu and redox‐substituted Pt layers on octahedral Au NPs was investigated to evaluate the electrocatalytic activity of Pt‐decorated octahedral NPs for the oxygen reduction reaction (ORR). By varying the number of cycles of Cu UPD and Pt replacement, the coverage by the Pt layer on the Au NPs could be easily controlled. The resulting Pt‐coated octahedral Au NPs exhibited three times higher specific activity for ORR than commercial Pt/C nanocatalysts due to the deposited Pt layer reflecting the {111} plane of the core. The ORR activity of Pt‐coated octahedral Au NPs could be enhanced by photoexcitation with monochromatic LED irradiation, and the increase in enhancement factor was even greater when irradiated with a wavelength that could efficiently excite the localized surface plasmon resonance (LSPR) of the NPs. Analysis shows that this improvement is due to improved diffusion of oxygen to the electrode.