Sub-monolayer Pt-modified Au dendrites (Au-Ds) were successfully synthesized and characterized using several techniques. The resulting Pt-modified Au-Ds effectively combined component catalytic properties and plasmonic optical properties. Results obtained from comprehensive kinetics experiments revealed that Pt-modified Au-Ds serve as an ideal photoelectocatalyst for the formic acid oxidation reaction (FAOR). Electro- and photoelectrocatalytic FAOR was carried out via the same dehydrogenation pathway on a high-efficiency Pt-modified Au-D surface with a low activation energy of 35.4 kJ mol(-1). The contribution from the photo-enhanced electrocatalytic effect reached 68% in the Tafel region. Photoelectrocatalysis and the photo-thermal effect synchronously boosted the FAOR on Pt-modified Au-Ds under illumination. Chronoamperometry results indicated that Pt-modified Au-Ds exhibit excellent stability and long-term durability for the FAOR.