Novel photocatalysts, g-C₃N₄/Co_xMo_1−xS₂ with different exposed facets of Co–Mo, were employed as catalysts for the examination of facet-dependent catalytic activity toward photocatalytic hydrogen evolution from water splitting with Eosin Y photosensitized as an antenna molecule under visible light irradiation. The studies showed that the g-C₃N₄/Co_xMo_1−xS₂ photocatalysts had higher activities than g-C₃N₄/MoS₂ and g-C₃N₄/CoS_x. The promoter Co atoms associated with Mo displayed synergistic effects and efficiently induced photo-generated charge carriers and electron transfer. A possible reaction mechanism for g-C₃N₄/Co_xMo_1−xS was corroborated using photo-luminescence spectra, photo-electro-chemical characterizations and electro-chemical impedance spectra studies. This study discloses the facet-dependent effect of metal–metal co-catalysts on semiconductor photo-catalysts in photo-catalytic water reduction, and will give an insight into the design and synthesis of highly efficient metal/semiconductor hybrid photocatalysts.