Single-atom catalysts (SACs) have shown unexpected catalytic activity due to their unique electronic structure and coordination environment. Nonetheless, the synthesis of an atomically precise low-coordination single-atom catalyst remains a grand challenge. Herein, we report a coordinately unsaturated Ni-N₃ single-atom electrocatalyst using a metal-organic framework (MOF) derived N-C support with abundant exposed N for excellent electrochemical CO₂ reduction. The obtained Ni-N₃/NC active site exhibited highly efficient CO₂-to-CO conversion with a Faradaic efficiency of 94.6% at the current density of 100 mA/cm². In situ X-ray absorption spectroscopy (XAS) measurement suggested that the Ni atomic center with unsaturated coordination had the lower initial chemical state and higher charge transfer ability. In situ Fourier transform infrared (FT-IR) and theoretical calculation results revealed that the unsaturated catalytically active center could facilitate activation of CO₂ and thus heighten CO₂ electroreduction activity. These findings provided insights into the rational design of definitive coordination structure of SACs for boosting activity and selectivity.