The design of photocatalysts for overall water splitting in visible light remains a huge challenge. Carbon quantum dots (CQDs)/Ag/Ag3PW12O40 nanocomposites have high photocatalytic activities for overall water splitting in visible light without electron acceptors or hole scavengers. In the present system, H2 evolution was performed on Ag particles. O2 evolution appears to be a result of the oxidation of H2O on the valence band of Ag3PW12O40, which is enhanced by promoted electron-transfer events at hybrid interfaces of the polyoxometalates and CQDs. The synergistic effects of CQDs, Ag, and Ag3PW12O40 enable the design of a water-splitting catalyst with a remarkably improved efficiency (with an active region of λ<650 nm; apparent quantum yield ≈4.9 % at 480 nm) and operational stability.