Platinum is a widely used precious metal in many catalytic nanostructures. Engineering the surface electronic structure of Pt-containing bi- or multimetallic nanostructure to enhance both the intrinsic activity and dispersion of Pt has remained a challenge. By constructing Pt-on-Au (Pt∧Au) nanostructures using a series of monodisperse Au nanoparticles in the size range of 2–14 nm, we disclose herein a new approach to steadily change both properties of Pt in electrocatalysis with downsizing of the Au nanoparticles. A combined tuning of Pt dispersion and its surface electronic structure is shown as a consequence of the changes in the size and valence-band structure of Au, which leads to significantly enhanced Pt mass-activity on the small Au nanoparticles. Fully dispersed Pt entities on the smallest Au nanoparticles (2 nm) exhibit the highest mass-activity to date towards formic acid electrooxidation, being 2 orders of magnitude (75–300 folds) higher than conventional Pt/C catalyst. Fundamental relationships correlating the Pt intrinsic activity in Pt∧Au nanostructures with the experimentally determined surface electronic structures (d-band center energies) of the Pt entities and their underlying Au nanoparticles are established.