Thiolate-protected gold nanoclusters (NCs) are promising catalytic materials for the electrochemical CO reduction reaction (CO RR). In this work an atomic level modification of a Au NC is made by substituting two surface Au atoms with two Cd atoms, and it enhances the CO RR selectivity to 90-95 % at the applied potential between -0.5 to -0.9 V, which is doubled compared to that of the undoped Au . Additionally, the Cd-doped Au Cd exhibits the highest CO RR activity (2200 mA mg at -1.0 V vs. RHE) among the reported NCs. This synergetic effect between Au and Cd is remarkable. Density-functional theory calculations reveal that the exposure of a sulfur active site upon partial ligand removal provides an energetically feasible CO RR pathway. The thermodynamic energy barrier for CO formation is 0.74 eV lower on Au Cd than on Au . These results reveal that Cd doping can boost the CO RR performance of Au NCs by modifying the surface geometry and electronic structure, which further changes the intermediate binding energy. This work offers insights into the surface doping mechanism of the CO RR and bimetallic synergism.