Efficient electrochemical splitting of water with exceptional durability can be a solution for growing global demand for energy. Herein, we systematically investigated the influence of the supporting two-dimensional (2D) substrate (rGO, g-C3N4, BCN, MoS2, and WS2) on the electrocatalytic performance of CoS2 nanoparticles (NPs) for overall water splitting. CoS2NPs decorated onto rGO, g-C3N4, BCN, MoS2, and WS2 sheets were synthesized by adopting a facile hydrothermal technique using cobalt salt and sulfur source as precursors. Compared to unsupported CoS2, the synthesized nanohybrids supported on 2D materials, namely, CoS2/rGO, CoS2/g-C3N4, CoS2/BCN, CoS2/MoS2, and CoS2/WS2 exhibited significantly higher water oxidation and reduction and overall water splitting efficiencies in 1.0 M KOH aqueous solution, with CoS2/MoS2 and CoS2/WS2 catalysts being the most effective ones. In the case of the hydrogen evolution reaction (HER), CoS2/WS2 performed as the best catalyst and was able to provide a current density of 10 mA cm-2 at an overpotential of 130 mV, whereas it only required 298 mV to generate the same current density for the oxygen evolution reaction (OER). The bifunctional nanohybrid CoS2/WS2 catalyst achieved a current density of 10 mA cm-2 over 96 h for the alkaline overall water splitting at a cell voltage of 1.6 V. Density functional theory (DFT) calculations were also performed to further describe and determine the HER catalytic performance of the studied catalysts. Comparing the absolute values of the studied catalysts' Gibbs free energy of adsorbed hydrogen intermediate, H∗, (GH∗), revealed that both CoS2/MoS2 and CoS2/WS2 hybrid catalysts exhibited the best surface stability and the highest HER catalytic performance.