In summary, the “pre-locking and nanoconfined polymerization” strategy enables the fabrication of ultrahigh-density Cu₃ multi-atom catalysts with precisely defined triangular active centers, elucidating how multi-atom sites promote the efficient and selective conversion of CO₂ to ethanol. Under sacrificial-agent-free conditions, this catalyst achieves 98% ethanol selectivity with a production rate of 117 μmol g^–1 h^–1. This work demonstrates how rational catalyst design at the atomic scale can precisely steer complex multi-electron transformations, offering general insights for improving the selective conversion of CO₂ into valuable multi-carbon products.