Photocatalysis is considered an ideal energy conversion technology that can convert solar energy into clean hydrogen energy through water splitting. Hydrogen can be used directly as a source of energy rather than fossil fuels due to its higher energy yield and eco-friendly attributes. However, most photocatalysts exhibit low photocatalytic performance due to their lower light absorption capability, higher energy bandgap, poor charge separation and migration capability, and rapid electron–hole pair recombination properties. To overcome the above-mentioned problems, in this study, we performed the in situ simultaneous photodeposition of ammonia-treated graphene oxide (NGO) on CTAB-assisted ZnIn₂S₄ (ZIS) for photocatalytic hydrogen evolution from splitting under visible light irradiation (450 nm). Amounts of 1 wt% of both Pt and triethanolamine were used as the co-catalyst and hole-scavengers, respectively. The reaction conditions (catalyst and co-catalyst loading, ammonia concentration and mixing, and reaction temperature and pH) were optimized. The NGO/ZIS composite increased H₂ production by 3.25-fold and 67-fold compared to pure ZIS and NGO, respectively, indicating the achievement of a synergetic effect. Synthesized photocatalysts were characterized by XRD, XPS, FTIR, SEM, TEM, BET, TRPL, DRS, PL, and EIS analysis. Density functional theory indicated that the free energy, dipole moment, HOMO-LUMO bandgap, chemical reactivity, and surface charge were significantly increased in NGO compared to pure graphene oxide. Due to the NGO/ZIS composite formation, the light-harvesting capability, energy bandgap, charge separation and migration, surface area and morphology, and electron–hole pair recombination properties were significantly improved.