Sub-15 nm Au nanoparticles have been fabricated on a nanostructured Ag surface at room temperature via a liquid-phase chemical deposition upon excitation of the localized surface plasmon resonance (SPR). Measurement of the SPR-mediated photothermal local heating of the substrate surface by a molecular thermometry strategy indicated the temperature to be above 230 degrees C, which led to an efficient decomposition of CH3AuPPh3 to form Au nanoparticles on the Ag surface. Particle sizes were tunable between 3 and 10 nm by adjusting the deposition time. A surface-limited growth model for Au nanoparticles on Ag is consistent with the deposition kinetics.