The hydrogen storage properties of metal nanoparticles change with particle size. For example, in a palladium–hydrogen system, the hydrogen solubility and equilibrium pressure for the formation of palladium hydride decrease with a decrease in the particle size, whereas hydrogen solubility in nanoparticles of platinum, in which hydrogen cannot be stored in the bulk state, increases. Systematic studies of hydrogen storage in Pd and Pt nanoparticles have clarified the origins of these nanosize effects. We found a novel hydrogen absorption site in the hetero-interface that forms between the Pd core and Pt shell of the Pd/Pt core/shell-type bimetallic nanoparticles. It is proposed that the potential formed in the hetero-interface stabilizes hydrogen atoms rather than interstitials in the Pd core and Pt shells. These results suggest that metal nanoparticles a few nanometers in size can act as a new type of hydrogen storage medium. Based on knowledge of the nanosize effects, we discuss how hydrogen storage media can be designed for improvement of the conditions of hydrogen storage.