In this paper, we report a high‐performance carbon nitride supported Cu single‐atom catalyst featuring defected low‐coordination Cu‐N2 motif (Cu‐N2‐V). Lead many recently reported photocatalysts and its Cu‐N3 and Cu‐N4 counterparts, Cu‐N2‐V exhibits superior photocatalytic activity for CO2 reduction to ethanol, delivering 69.8 μmol g‐1 h‐1 ethanol production rate, 97.8% electron‐based ethanol selectivity, and a yield of ~10 times higher than Cu‐N3 and Cu‐N4. Revealed by the extensive experimental investigation combined with the DFT calculation, the superior photoactivity of Cu‐N2‐V stems from its unique defected Cu‐N2 configuration. Firstly, Cu in Cu‐N2‐V exist in Cu+/Cu2+ dual valence states, although predominantly in Cu+. The Cu+ sites support CO2 activation and the Cu+/Cu2+ sites are conducive for strong *CO adsorption and subsequent *CO‐*CO dimerization enabling C‐C coupling. Secondly, the Cu sites in Cu‐N2‐V are rich in electrons and thus highly active. Together they dictate the rate‐determining step on CO2 photoreduction to ethanol and lower the Gibbs free energy change. Furthermore, the defected configuration also promotes light adsorption and charge separation efficiency. Collectively, these make Cu‐N2‐V an effective and high‐performance catalyst for solar‐driven CO2 conversion to ethanol. This study also reveals the valence state change of Cu in Cu‐N2‐V during the CO2 photoreduction reaction.