The exploration of highly active and stable noble-metal-free electrocatalysts for hydrogen and oxygen evolution reaction is a challenging task to achieve sustainable production of H2 through water splitting. Herein, we present the design and synthesis of a novel three-dimensional(3D)-networked heterogeneous nickel phosphide/sulfide electrocatalyst consisting of Ni2P strongly coupled with Ni3S2 in situ grown on Ni foam. Benefiting from the strong interfacial coupling effects between Ni2P and Ni3S2, large surface area, highly conductive Ni foam support, and the unique 3D open configuration, the optimal 3D-networked hybrid electrode exhibits superior electrocatalytic activity with extremely low overpotentials of 80 and 210 mV to deliver a current density of 10 mA cm−2 for HER and OER in 1.0 M KOH, respectively. Assembled as an electrolyzer for overall water splitting, this electrode delivers an impressive low onset potential of only 1.45 V and gives a current density of 10 mA cm−2 at a very low cell voltage of 1.50 V, which is dramatically superior to the current state-of-the-art electrocatalysts. In combination with density functional theory (DFT) calculations, this study demonstrates that the strong coupling interactions between Ni2P and Ni3S2 synergistically optimize the electronic structure and tune the hydrogen (or water) adsorption energy, thus significantly enhancing the overall electrochemical water-splitting activity. Our work might shed some new lights on the design and fabrication of efficient and robust three-dimensional hybrid electrode materials for a variety of electrochemical applications.