We have succeeded in depositing monodisperse 1 nm Pt nanoparticles (NPs) on cadmium sulfide (CdS) nanostructures by employing a facile wet-chemical method. This approach is applicable for various CdS nanostructures regardless of their size, shape, and crystal structure. Moreover, photocatalytic measurements revealed that the Pt NP-deposited CdS NPs showed higher activity than those synthesized by photodeposition did. The mechanism of Pt NP deposition on CdS was investigated using transmission electron microscopy, X-ray diffraction analysis, X-ray fluorescence analysis, and absorption spectroscopy.