Photocatalytic CO₂ reduction on metal-oxide-based catalysts is promising for solving the energy and environmental crises faced by mankind. The oxygen vacancy (V_o) on metal oxides is expected to be a key factor affecting the efficiency of photocatalytic CO₂ reduction on metal-oxide-based catalysts. Yet, to date, the question of how an V_o influences photocatalytic CO₂ reduction is still unanswered. Herein, we report that, on V_o-rich gallium oxide coated with Pt nanoparticles (V_o-rich Pt/Ga₂O₃), CO₂ is photocatalytically reduced to CO, with a highly enhanced CO evolution rate (21.0 μmol·h^-1) compared to those on V_o-poor Pt/Ga₂O₃ (3.9 μmol·h^-1) and Pt/TiO₂(P25) (6.7 μmol·h^-1). We demonstrate that the V_o leads to improved CO₂ adsorption and separation of the photoinduced charges on Pt/Ga₂O₃, thus enhancing the photocatalytic activity of Pt/Ga₂O₃. Rational fabrication of an V_o is thereby an attractive strategy for developing efficient catalysts for photocatalytic CO₂ reduction.