Electrochemical reduction of CO₂ to chemical fuel offers a promising strategy for managing the global carbon balance, but presents challenges for chemistry due to the lack of effective electrocatalyst. Here we report atomically dispersed nickel on nitrogenated graphene as an efficient and durable electrocatalyst for CO₂ reduction. Based on operando X-ray absorption and photoelectron spectroscopy measurements, the monovalent Ni(i) atomic center with a d⁹ electronic configuration was identified as the catalytically active site. The single-Ni-atom catalyst exhibits high intrinsic CO₂ reduction activity, reaching a specific current of 350 A g_catalyst⁻¹ and turnover frequency of 14,800 h⁻¹ at a mild overpotential of 0.61 V for CO conversion with 97% Faradaic efficiency. The catalyst maintained 98% of its initial activity after 100 h of continuous reaction at CO formation current densities as high as 22 mA cm⁻².