A suitable photocatalyst for overall water splitting has been produced by overcoming the disadvantage of the band structure in bulk BiOCl by reducing the thickness to the quantum scale. The ultrathin BiOCl nanosheets with surface/subsurface defects realized the solar-driven pure water splitting in the absence of any co-catalysts or sacrificial agent. These surface defects cannot only shift both the valence band and conduction band upwards for band-gap narrowing but also promote charge-carrier separation. The amount of defects in the outer layer surface of BiOCl results in an enhancement of carrier density and faster charge transport. First-principles calculations provide clear evidence that the formation of surface oxygen vacancies is easier for the ultrathin BiOCl nanosheets than for its thicker counterpart. These defects can serve as active sites to effectively adsorb and dissociate H2 O molecules, resulting in a significantly improved water-splitting performance.