[en] Two-dimensional photocatalysts for water splitting with spontaneous hydrogen and oxygen evolution half reactions are long-desired materials for solving the environmental pollution and energy crisis. To date, several such photocatalysts have been reported, all based on metal-containing materials. Here, we propose a novel metal-free candidate, SiP₂S₆ monolayer, on the basis of first principles calculations. SiP₂S₆ monolayer holds a moderate band gap of 2.78 eV and suitable band edge positions for overall water splitting. Also, it harbors excellent optical absorption, even comparable to that of g-C₃N₄ monolayer. More remarkably, the photocatalytic hydrogen and oxygen evolution half reactions on the surface of SiP₂S₆ monolayer can occur spontaneously with only solar light irradiation, without needing sacrificial reagents or co-catalysts. This discovery not only reveals the great potential of SiP₂S₆ monolayer in photocatalytic applications but also broadens the scientific impact of two-dimensional photocatalysts. (paper)