The use of single-atom cocatalysts plays a crucial role in enhancing artificial photocatalysis, where the precise construction of stable and efficient single-atom configuration is essential but remains challenging. Here, we report a simple one-step hydrothermal method for preparing single-atomic Mo modified ZnIn₂S₄ (Mo-ZIS) nanosheets as a highly active photocatalytic hydrogen evolution (PHE) photocatalyst. The Mo substituting for portion of In atoms in ZIS nanosheets induces the spatial charge redistribution, which not only promotes the separation of photogenerated charge carriers but also optimizes the Gibbs free energy of adsorbing H* on S atoms at basal planes. As a result, Mo-ZIS exhibits an impressive PHE rate as high as 6.71 mmol·g⁻¹·h⁻¹, over 10 times that of the pristine ZIS, with an apparent quantum efficiency (AQE) up to 38.8% at 420 nm. This study gains insights into the coordination configuration and electronic modulation resulting from single-atomic decoration, providing mechanistic cognitions for the development of advanced photocatalysts via non-precious metal atomic modification.