The reactivity of zirconia nanoparticles has been investigated by means of DFT+U calculations as a function of the morphology and stoichiometry. For comparative purposes, a single Au atom has been deposited on the stoichiometric and O-deficient regular (101) surface, on the stepped (156) surface, and on nanoparticles in the range of 0.9–1.9 nm in size. We show that, under stoichiometric conditions, nanostructuring leads to enhanced binding energies and redox processes with the supported metal that are not found on the extended surfaces. These new features are due to the structural flexibility and peculiar electronic structure displayed by the nanoparticles. In this respect, nanostructuring of oxide supports can modify and possibly improve the catalytic activity of the deposited metals. In contrast, we show that under reducing conditions nanostructuring stabilizes the O vacancies making zirconia nanoparticles less reactive toward Au adsorption than O-deficient extended surfaces.