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Modulating the local coordination structure of metal single-atom catalysts (SACs) is extensively employed to tune the catalytic activity, but rarely involved in regulating the reaction pathway which fundamentally determines the product selectivity. Herein, we report that the product selectivity of electrochemical CO2 reduction (CO2RR) on the single-atom indium-NxC4−x (1 ≤ x ≤ 4) catalysts could be tuned from formate to CO by varying the carbon and nitrogen occupations in the first coordination sphere. Surprisingly, the optimal In SAC showed great promise for CO production with the maximum Faradic efficiency of 97%, greatly different from the reported In-based catalysts where the formate is the dominant product. Combined experimental verifications and theoretical simulations reveal that the selectivity switch from formate to CO on In SACs originates from active sites shift from indium center to the indium-adjacent carbon atom, where the indium site favors formate formation and the indium-adjacent carbon site prefers the CO pathway. The present work suggests the active sites in metal SACs may shift from the widely accepted metal center to surrounding carbon atoms, thereby offering a new implication to revisit the active sites for metal SACs. [ABSTRACT FROM AUTHOR]