Water adsorption and dissociation on clean and oxygen pre-covered Ni(111) surfaces have been computed systematically by using density functional theory and ab initio atomistic thermodynamics. The adsorption of H, O and OH prefers 3-fold faced centered cubic hollow sites, and that of H₂O prefers the top site. For (H₂O)_n aggregation, direct O–Ni interaction and H-bonding synergistically determine the adsorption energy, which is structure insensitive for large adsorbed clusters. At low coverage (θ ≤ 0.25 ML), OH adsorbs perpendicularly and prefers remote distribution without H-bonding, and the OH saturation coverage should be 0.625 ML on the basis of H₂O dissociative adsorption. The adsorption configuration for 4O (0.25 ML) prefers a p(2 × 2) structure, in agreement with the experiment, and the O saturation coverage should be 0.25 ML based on H₂O dissociative adsorption. On the 0.25 ML O pre-covered Ni(111), H₂O greatly prefers molecular adsorption [4O + 4H₂O(s)] over dissociative adsorption [8OH] thermodynamically, and this is in agreement with the photoelectron spectroscopy results and in disagreement with previously and recently proposed results from single crystal adsorption calorimetry of D₂O adsorption. It is noted that the computed bond energy and formation enthalpy of the supposed surface hydroxyls on the basis of the molecular adsorbed state [4O + 4H₂O(s)] are much closer to the experimentally estimated results than those computed on the basis of the dissociatively adsorbed state [8OH]. Furthermore, the computed H₂O desorption temperature on the basis of the molecular adsorbed state [4O + 4H₂O(s)] is in excellent agreement with experimental results (284 vs. 275–300 K), while that from the dissociatively adsorbed state [8OH] differs strongly (197 K). All these support H₂O molecular adsorption instead of dissociative adsorption, and this needs further experimental investigations and confirmations. The vibrational frequencies of 4O, 4O + 4H₂O and 8OH adsorption configurations have been computed to aid experimental studies.