Strain engineering offers an attractive strategy for improving intrinsic catalytic performance of a heterogeneous catalyst. Herein, we successfully create strain into layered indium sulfide (In₂S₃) at atomic scale via introducing oxygen coordination and sulfur vacancy using a wet-chemistry method. The atomically strained In₂S₃ exhibits greatly enhanced CO₂ photoreduction performance, achieving a CO₂ to CO conversion rate of 5.16 μmol g_catalyst⁻¹ h⁻¹ under visible light illumination in ambient air. In-situ spectroscopic measurements together with theoretical calculations indicate that the atomically strained In₂S₃ features lattice disordered defects on surface, which provides rich uncoordinated catalytic sites and induces structural distortion, resulting in modified band structure that promotes CO₂ adsorption/activation and boosts photogenerated charge carriers’ separation during CO₂ photoreduction. This work provides a new approach for the rational design of atomically strained photocatalysts for CO₂ reduction in ambient air.