ZnIn₂S₄–g-C₃N₄ sheet-on-sheet nanocomposites with different g-C₃N₄ contents were synthesized by a facile hydrothermal method and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), high-resolution transmission electron microcopy (HRTEM), N₂ adsorption–desorption, ultraviolet-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) experiments. The photocatalytic activities of these samples were evaluated by the photocatalytic H₂-production and degradation of organic pollutants (methyl orange and phenol) under visible-light illumination (λ > 420 nm). The results showed that the ZnIn₂S₄–g-C₃N₄ composite photocatalysts displayed higher photocatalytic activity than the pristine g-C₃N₄ and ZnIn₂S₄ both for H₂-evolution and degradation of pollutants. The optimal g-C₃N₄ content was determined to be 40 wt%, and the corresponding H₂-production rate was 953.5 μmol h⁻¹ g⁻¹, which was about 1.91 times higher than that of pure ZnIn₂S₄. The enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites should be attributed to the well-matched band structure and intimate contact interfaces between ZnIn₂S₄ and g-C₃N₄, which led to the effective transfer and separation of the photogenerated charge carriers. Moreover, the ZnIn₂S₄–g-C₃N₄ composites showed excellent stability during the photocatalytic reactions under visible light. A possible mechanism of the enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites was proposed and supported by the PL and PEC results.