The effects of intrinsic defects in monoclinic bismuth vanadate (BiVO4) on its stability and optoelectronic properties for photochemical water splitting application were examined using density functional theory. Among the most favorable structures, only that associated with V-antisites on Bi with additional Bivacancies (Bi(1–5x)V(1+3x)O4 with x = 0.0625) revealed narrower band gap energy by 0.5 eV compared to pristine material (calculated value is 2.8 eV) giving a value of 2.3 eV, which is very close to the experimentally reported ones (in the 2.4–2.5 eV range). The low electron mobility reported experimentally for this material was also confirmed by the relatively large electron effective masses obtained for the intrinsic defective Bi(1–5x)V(1+3x)O4 (x = 0.0625) structure along the three principal crystallographic directions. The strongly localized nature of the accommodated electrons on the d-orbitals of the newly substituted V at Bi sites was also predicted to be at the origin of the poor H2 evolution performance of this material.