We examined the water adsorption and dissociation on ceria surfaces as well as ceria-supported Au single-atom catalysts using density functional theory calculations. Molecular and dissociative water were observed to coexist on clean CeO₂ and reduced Au₁/CeO_2−x surfaces because of the small difference in adsorption energies, whereas the presence of dissociative water was highly favorable on reduced CeO_2−x and clean Au₁/CeO₂ surfaces. Positively charged Au single atoms on the ceria surface not only provided activation sites for water adsorption but also facilitated water dissociation by weakening the intramolecular O–H bonds. In contrast, negatively charged Au single atoms were not reactive for water adsorption because of the saturation of Au 5d and 6s electron shells. This work provides a fundamental understanding of the interaction between water and single-atom Au catalysts.