Understanding the mechanism of water dissociation on metal oxide surfaces is of particular interest in catalytic reactions. In this work, the interaction of water with the ZnO(0001) polar surface is investigated, and the role of oxygen adatoms in water splitting is uncovered. The individual surface energies and electronic properties of ZnO polar surfaces are investigated on the basis of density functional theory calculations. The oxygen adatoms on the ZnO(0001) surface introduce in-gap surface states, resulting in a direct-to-indirect band gap transition. Water strongly interacts with oxygen adatoms to spontaneously form hydroxyl groups, recovering the direct band-gap characteristics of ZnO polar surfaces. Furthermore, water prefers to adsorb at the step edges of cavities after all of the oxygen adatoms are consumed, and the hydrogen-bonded network among water molecules triggers the dissociation of water at the edge sites, which is also confirmed by molecular dynamics calculations. Our results provide ato...