The substoichiometric {111}, {110}, and {100} surfaces of UO₂ and PuO₂ are studied computationally using two distinct yet related approaches based on density functional theory (DFT): the periodic electrostatic embedded cluster method and Hubbard-corrected periodic boundary condition DFT. The first and the second layer oxygen vacancy formation energies and geometries are presented and discussed; the energies are found to be substantially larger for UO₂ versus PuO₂, a result that traced to the substantially more positive An(IV)/An(III) reduction potential for Pu and hence relative ease of Pu(III) formation. For {110} and {100} surfaces, the significantly more stable dissociative water adsorption seen previously for stoichiometric surfaces (J. Nucl. Mater. 2016, 482, 124–134; J. Phys. Chem. C 2017, 121, 1675–1682) is also found for the defect surfaces. By contrast, the vacancy creation substantially changes the most stable mode of water adsorption on the {111} surface, such that the almost degenerate molecular and dissociative adsorptions on the pristine surface are replaced by a strong preference for dissociative adsorption on the substoichiometric surface. The implications of this result for the formation of H₂ are discussed. The generally very good agreement between the data from the embedded cluster and periodic DFT approaches provides additional confidence in the reliability of the results and conclusions.