Engineering the electronic properties of semiconductor-based photocatalysts using elemental doping is an effective approach to improve their catalytic activity. Nevertheless, there still remain contradictions regarding the role of the dopants played in photocatalysis. Herein, ultrathin ZnIn2S4 (ZIS) nanosheets with oxygen doping were synthesized by a one-pot solvothermal method. XRD, XPS and Raman spectral measurements support the presence of lattice oxygen in the oxygen-doped ZIS (OZIS) sample. With optimum doping of oxygen, the ultrathin OZIS nanosheets show enhanced CO2-to-CO conversion activity with a CO-evolving rate of 1680 μmol h−1 g−1 under visible light irradiation, which is about 7 times higher than that of the pristine ZIS. First-principle calculations support that doping of oxygen in the lattice of ZnI2S4 nanosheets plays a key role in tuning its electronic properties. The remarkable photocatalytic performance of OZIS can be assigned to a synergistic consequence of a unique ultrathin-layered structure and an upward shift of the conduction band minimum (CBM) caused by the oxygen doping into ZIS and the quantum confinement effect (QCE) induced by the decreased particle size after doping as well as to the improved charge separation efficiency. The present work offers a simple elemental doping method to promote charge separation at atomic level and illustrates the roles played by oxygen doping in photocatalysis, giving new insights into highly efficient artificial photosynthesis.