The advancement of effective and robust catalysts that can replace the precious-metal-based benchmarks for electrocatalytic overall water splitting is a highly fashionable approach to exploring sustainable energy utilization and addressing environmental issues. Herein, we demonstrate a facile single-step fabrication of NiS-NiS2 nanoparticles in situ grown on the layered porous sulfur-doped graphitic carbon nitride nanosheets (NiS-NiS2/SGCN) act as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium.Further, an increasing the Ni precursor (fixed sulfur source) increases the formation of NiS-NiS2 on the surface of SGCN (labeled as NS-1.0, NS-2.0, and NS-3.0). Structural and morphological studies such as PXRD, XPS, FTIR, HR-TEM, etc., revealed a uniform distribution of NiS/NiS2 on the surface of S-g-C3N4 nanosheets. The OER and HER activity of NiS-NiS2/SGCN nanohybrid were significantly enhanced by increasing the affinity of accessible surface-active edge sites, interfacial contact allows effective modification of electronic structure, high structural porosity, redox-active centers of Ni2+/Ni3+, and rich in sulfur vacancy. In addition, the enhancement of the exposure of the edge sites is improved by their attachment to a sulfur-doped g-C3N4 framework. As a result, in the half-cell experiments, the NiS-NiS2/SGCN (NS-3.0) catalyst exhibited enhanced performance in both OER and HER, requiring an overpotential of 298 mV to reach 50 mA/cm2 for OER and −144 mV to reach 10 mA/cm2 for HER. Furthermore, the constructed electrolyzer using Ni-S 3.0 as the cathode and anode required a cell voltage of 1.66 V to yield 50 mA/cm2 in overall water splitting. This study may inspire the in-situ synthesis of different metal sulfides on SGCN to provide a unique interfacial approach for improving the performance of bifunctional non-precious electrocatalysts.