Water is often involved in many catalytic processes, which can strongly affect structural evolution of catalysts during pretreatments and catalytic reactions. In this work, we demonstrate a promotional effect of H2O on both oxidative dispersion and spontaneous aggregation of Ag nanocatalysts supported on alumina. Ag nanoparticles supported on γ-Al2O3 and Ag nanowires on Al2O3(0001) can be dispersed into nanoclusters via annealing in O2 above 300 °C, which is accelerated by introduction of H2O into the oxidative atmosphere. Furthermore, the formed highly dispersed Ag nanoclusters are subject to spontaneous aggregation in humid atmosphere at room temperature. Ex situ and in situ characterizations in both powder and model catalysts suggest that formation of abundant surface hydroxyls and/or water adlayer on the Al2O3 surface in the H2O-containing atmosphere facilitates the surface migration of Ag species, thus promoting both dispersion and aggregation processes. The aggregation of the supported Ag nanostructures induced by the humid oxidative atmosphere enhances CO oxidation but inhibits selective catalytic reduction of NO with C3H6. This work illustrates the critical role of H2O in structure and catalytic performance of metal nanocatalysts, which can be widely present in heterogeneous catalysis.