Abstract Achieving CO 2 reduction with H 2 O on metal photocatalysts and understanding the corresponding mechanisms at the molecular level are challenging. Herein, we report that quantum-sized Au nanoparticles can photocatalytically reduce CO 2 to CO with the help of H 2 O by electron-hole pairs mainly originating from interband transitions. Notably, the Au photocatalyst shows a CO production rate of 4.73 mmol g −1 h −1 (~100% selectivity), ~2.5 times the rate during CO 2 reduction with H 2 under the same experimental conditions, under low-intensity irradiation at 420 nm. Theoretical and experimental studies reveal that the increased activity is induced by surface Au–O species formed from H 2 O decomposition, which synchronously optimizes the rate-determining steps in the CO 2 reduction and H 2 O oxidation reactions, lowers the energy barriers for the *CO desorption and *OOH formation, and facilitates CO and O 2 production. Our findings provide an in-depth mechanistic understanding for designing active metal photocatalysts for efficient CO 2 reduction with H 2 O.