Exploring appropriate methods and agents for the fabrication of heterostructures opens up a new avenue for photocatalytic water splitting by utilizing solar power as an energy source. In this work, heterostructured photocatalysts consisting of ultrathin graphitic carbon nitride (g-C₃N₄) nanolayers decorated with MoS₂ (g-C₃N₄/MoS₂) were prepared through a conventional hydrothermal method with N,N-dimethylformamide (DMF) as the solvent, which prohibited the morphology damage of g-C₃N₄ nanosheets under high temperature and pressure. The achieved nanosized few-layer MoS₂ with abundant defect sites and disordered structures was uniformly grown on the g-C₃N₄ nanosheets. The structures and optical properties of g-C₃N₄/MoS₂ heterostructures were carefully characterized. By optimizing the proportion of MoS₂ decoration, the g-C₃N₄/MoS₂ heterostructure with a MoS₂ content of 2.5 wt% could exhibit the highest photocatalytic H₂ evolution activity under visible-light-irradiation (λ ≥ 400 nm) of 732.0 μmol g⁻¹ h⁻¹, which was 58.6 fold more than that of undecorated g-C₃N₄ (12.5 μmol g⁻¹ h⁻¹). Photoelectrochemical analysis and photoluminescence (PL) results demonstrated the existence of interfacial charge transfer in g-C₃N₄/MoS₂ heterostructures, which leads to efficient separation of photogenerated electron–hole pairs. Finally, electron transfer kinetics from g-C₃N₄ to MoS₂ were systematically investigated, and g-C₃N₄/MoS₂-2.5 wt% (CM-2.5) presented the fastest electron injection rate (2.2 × 10⁻¹⁰ s⁻¹) and highest efficiency (43.4%), which were responsible for the enhanced photocatalytic H₂ evolution.