The reaction mechanism of H2 dissociative adsorption on WO3 surfaces was studied by a first-principles method. Calculations for the clean surface indicated that the c(2×2) reconstruction was the most stable surface geometry. Four H2 dissociative adsorption models were investigated. The optimal configuration was for two H atoms adsorbed at the terminal O1c site, followed by water formation and an oxygen vacancy on the surface. The density of states (DOS) results revealed that H2 dissociative adsorption led to partial filling of the conduction band, which accounted for the increase of WO3 electrical conductivity upon H2 exposure.