Recent years have seen a surge of interest in tuning the optical properties of metals for a wide range of applications. In contrast to the well-studied plasmonic metals (mainly Au and Ag), which have distinct absorption peaks, tuning the absorption peak of small (<10 nm) Pt nanoparticles in the visible spectral region, but without increasing their size, has been a major challenge. Here we report, for the first time, a new light absorption model to modulate the absorption peak of supported small Pt nanoparticles in the visible spectral region by adjusting their dielectric environment instead of changing their size. In this model, the Pt nanoparticles can absorb the scattered light in the near field of the dielectric surface of a spherical SiO2 support, thereby exhibiting well-defined visible-light absorption peaks and driving photocatalytic redox reactions. This discovery could open a promising new route to using Pt nanoparticles as visible-light photon absorbers for solar energy conversion.