AbstractDirect experimental observations of the interface structure can provide vital insights into heterogeneous catalysis. Examples of interface design based on single atom and surface science are, however, extremely rare. Here, we report Cu–Sn single-atom surface alloys, where isolated Sn sites with high surface densities (up to 8%) are anchored on the Cu host, for efficient electrocatalytic CO₂ reduction. The unique geometric and electronic structure of the Cu–Sn surface alloys (Cu₉₇Sn₃ and Cu₉₉Sn₁) enables distinct catalytic selectivity from pure Cu₁₀₀ and Cu₇₀Sn₃₀ bulk alloy. The Cu₉₇Sn₃ catalyst achieves a CO Faradaic efficiency of 98% at a tiny overpotential of 30 mV in an alkaline flow cell, where a high CO current density of 100 mA cm⁻² is obtained at an overpotential of 340 mV. Density functional theory simulation reveals that it is not only the elemental composition that dictates the electrocatalytic reactivity of Cu–Sn alloys; the local coordination environment of atomically dispersed, isolated Cu–Sn bonding plays the most critical role.