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Solar-driven transfer hydrogenation of unsaturated bonds has received considerable attention in the research area of sustainable organic synthesis; however, water, the ultimate green source of hydrogen, has rarely been investigated due to the high barrier associated with splitting of water molecules. We report a carbon-nitride-supported palladium single-atom heterogeneous catalyst with unparalleled performance in photocatalytic water-donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic-labeling experiments and operando nuclear magnetic resonance measurements confirm the direct hydrogenation mechanism using in situ-generated protons from water splitting under visible-light irradiation. Density functional theory calculations attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the atomic palladium sites.