AbstractAtomic-level coordination engineering is an efficient strategy for tuning the catalytic performance of single-atom catalysts (SACs). However, their rational design has so far been plagued by the lack of a universal correlation between the coordination symmetry and catalytic properties. Herein, we synthesised planar-symmetry-broken CuN₃ (PSB-CuN₃) SACs through microwave heating for electrocatalytic CO₂ reduction. Remarkably, the as-prepared catalysts exhibited a selectivity of 94.3% towards formate at −0.73 V vs. RHE, surpassing the symmetrical CuN₄ catalyst (72.4% at −0.93 V vs. RHE). In a flow cell equipped with a PSB-CuN₃ electrode, over 90% formate selectivity was maintained at an average current density of 94.4 mA cm⁻² during 100 h operation. By combining definitive structural identification with operando X-ray spectroscopy and theoretical calculations, we revealed that the intrinsic local symmetry breaking from planar D_4h configuration induces an unconventional dsp hybridisation, and thus a strong correlation between the catalytic activity and microenvironment of metal centre (i.e., coordination number and distortion), with high preference for formate production in CuN₃ moiety. The finding opens an avenue for designing efficient SACs with specific local symmetries for selective electrocatalysis.