The effects of hydration and oxygen vacancy on CO2 adsorption on the beta-Ga2O3(100) surface have been studied using density functional theory slab calculations. Adsorbed CO2 is activated on the dry perfect beta-Ga2O3(100) surface, resulting in a carbonate species. This adsorption is slightly endothermic, with an adsorption energy of 0.07 eV. Water is preferably adsorbed molecularly on the dry perfect beta-Ga2O3(100) surface with in adsorption energy of -0.56 eV, producing a hydrated perfect beta-Ga2O3(100) surface. Adsorption of CO2 on the hydrated surface as a carbonate species is also endothermic, with an adsorption energy of 0.14 eV. indicating a slightly repulsive interaction when H2O and CO2 are coadsorbed. The carbonate species on the hydrated perfect surface can be protonated by the coadsorbed H2O to a bicarbonate species. making the CO2 adsorption exothermic, with an adsorption energy of -0.13 eV. The effect of defects on CO2 adsorption and activation has been examined by creating an oxygen vacancy on the dry beta-Ga2O3(100) surface. The formation of an oxygen vacancy is endothermic, by 0.34 eV. with respect to a free O-2 molecule in the gas phase. Presence of the oxygen vacancy promoted the adsorption and activation of CO2. In the most stable CO2 adsorption configuration on the dry defective beta-Ga2O3(100) surface with an oxygen vacancy, one of the oxygen atoms of the adsorbed CO2 occupies the oxygen vacancy site and the CO2 adsorption energy is -0.31 eV. Water favors dissociative adsorption at the oxygen vacancy site on the defective surface. This process is spontaneous, with a reaction energy of -0.62 eV. These results indicate that. when water and CO2 are present in the adsorption system simultaneously. water will compete with CO2 for the oxygen vacancy sites and impact CO2 adsorption and conversion negatively.