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Au is one of the most promising electrocatalysts to convert CO 2 into CO in an aqueous-phase electrochemical reduction. However, ultrasmall Au nanocatalysts (AuNCs, <2 nm) have proven to be favorable for water reduction over CO 2 , although they possess a large surface-to-volume ratio and potentially are ideal for CO 2 reduction. We herein report that ultrasmall AuNCs (1.9 ± 0.3 nm) supported on nitrided carbon are remarkably active and selective for CO 2 reduction. The mass activity for CO of AuNCs reaches 967 A g −1 with a faradaic efficiency for CO of ∼83% at −0.73 V ( vs . reversible hydrogen electrode) that is an order of magnitude more active than the state-of-the-art results. The high activity is endowed by the large surface area per unit weight and the high selectivity of ultrasmall AuNCs for CO 2 reduction originates from the cooperative effect of Au and the nitrided carbon support where the surface N sites act as Lewis bases to increase the surface charge density of AuNCs and enhance the localized concentration of CO 2 nearby catalytically active Au sites. We show that our results can be applied to other pre-synthesized Au catalysts to largely improve their selectivity for CO 2 reduction by 50%. Our method is expected to illustrate a general guideline to effectively lower the cost of Au catalysts per unit weight of the product while maintaining its high selectivity for CO 2 reduction.