Metal cluster structures composed of tens of atoms represent a new class of catalysts with potentially superior catalytic activity and durability compared with those of other conventional catalysts. However, controlling the number of constituent atoms in clusters and achieving high-density loading on supports remain challenging. We reduce PtCl42− with methanol to create platinum atoms forming strongly anchored platinum clusters with controlled atom counts on selected surfaces of alumina. We resolved the atomic structures of the platinum clusters and could correlate the degree of cluster-support interaction, catalytic activity, and durability with atom counts. We demonstrate the promise of this approach by developing platinum clusters that exhibit the highest catalytic performance per platinum usage reported to date for hydrogen production from the dehydrogenation of cyclic hydrocarbon hydrogen carriers.