The options of transition metals as co-catalysts for photocatalytic H2S splitting are restricted to some noble metals and related compounds which have noticeable achievements despite their high prices. Substituting with cheap transition metals and downsizing the size to single atom level are economic ways to lower the cost. Herein, the s-triazine graphite-like carbon nitride sheet g-C3N4 (001) is chosen as the model to study the performances of 3d and 4d transition metal single atoms (TMSA = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd) in H2S splitting based on density functional theory (DFT) calculations. It is found that low-cost transition metals with industrial relevance (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Cd) are completely comparable with noble metals (Ru, Rh, Pd, Ag). Among them, V and Nb are the most promising co-catalysts with good thermodynamic stabilities, favorable responses to visible light, high photoinduced electron-hole separation efficiencies, sufficient potentials for H2S splitting, and low energy barriers for H2S dissociation into H-2 and S. The noticeable improved activities of V/g-C3N4 and Nb/g-C3N4 are attributed to the formation of strong interfacial chemical bonds which could promote electrons transferring to H2S derivates. In addition, the introduction of photoinduced electrons could further improve the activities of V/g-C3N4 and Nb/g-C3N4 with more electrons transferring to H2S derivates. It is expected that this work could provide a helpful guidance to choose appropriate TMSA co-catalysts as references for H2S splitting. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.