Single-atom catalysts (SACs) have attracted increasing concerns in electrocatalysis because of their maximal metal atom utilization, distinctive electronic properties, and catalytic performance. However, the isolated single sites are disadvantageous for reactions that require simultaneously activating different reactants/intermediates. Fully exposed metal cluster catalyst (FECC), inheriting the merits of SACs and metallic nanoparticles, can synergistically adsorb and activate reactants/intermediates on their multi-atomic sites, demonstrating great promise in electrocatalytic reactions. Here a facile method to regulate the atomic dispersion of Ni species from cluster to single-atom scale for efficient CO₂ reduction was developed. The obtained Ni FECC exhibits high Faradaic efficiency of CO up to 99%, high CO partial current density of 347.2 mA cm⁻², and robust durability under 20 h electrolysis. Theoretical calculations illuminate that the ensemble of multiple Ni atoms regulated by sulfur atoms accelerates the reaction kinetics and thus improves CO production.