Nickel sulfide nanoparticles were successfully grown as a cocatalyst on the surface of polymeric g-C3N4 photocatalysts through a hydrothermal method. The NiS2 composition is confirmed by various spectroscopic techniques and electron microscopy. It was found that the presence of NiS2 nanoparticles on the g-C3N4 surface could greatly enhance the photocatalytic activity of g-C3N4 for hydrogen generation under visible-light irradiation. Significantly, the NiS2-loaded g-C3N4 was capable of showing an even higher photocatalytic H2 generation rate than that of Pt-loaded g-C3N4. Such enhanced photocatalytic activities by NiS2-loading could be attributed to the effective charge transfer between g-C3N4 and the attached NiS2 nanoparticles which might also serve as active sites for proton reduction into H2. Our studies demonstrate a promising strategy to develop economic noble-metal-free composites as photocatalysts for efficient solar-to-hydrogen conversion.