The development of photocatalysts with synergetic bifunctional catalytic centers is of great importance for efficient photocatalytic water splitting towards sustainable conversion and storage of renewable solar energy. Here, we present a conceptual design of atomic-level bifunctional redox single sites to realize an efficient catalytic reaction of photoexcited electron–hole pairs on g-C₃N₄-based photocatalysts for spontaneous overall water splitting. The atomically dispersed heterogeneous Pt₁N_x–Au₁N_x redox single sites anchored on a g-C₃N₄ support could successfully introduce p–n coupled micro-domains as effective bifunctional electron-donor–acceptor centers, which significantly improves the electron–hole separation efficiency to ∼60% in the visible-light region of 400–450 nm which is 6–10 times that of individual Pt/C₃N₄ and Au/C₃N₄. The bifunctional Pt–Au/C₃N₄ single-site photocatalyst could thus effectively split pure water with a reduced surface redox reaction barrier by ∼300 mV, resulting in ∼10-fold enhancement of the photocatalytic performance.