Transition metal nitrides (TMNs) are considered to be some of the most promising metallic materials for electrocatalytic water splitting. However, the low density of active sites and weak reaction kinetics still limit their wide industrial application. Herein, we put forward a typical 3D hierarchical heterostructure that is composed of metallic Ni3N, Mo5N6, and Ni grown on nickel foam (denoted as Ni3N@NiMoNx/NF), presenting it as a highly-active bifunctional electrocatalyst for water splitting. This hybrid nanowire heterojunction has an abundant interface structure for more catalytically active sites, while its synergistic effects of strong electronic interaction and intrinsic high conductivity ensure fast electron transfer for rapid reaction kinetics. Remarkably, the Ni3N@NiMoNx/NF electrode delivers small overpotentials of 78 mV and 370 mV at 100 mA cm-2 for the HER and OER, respectively. By utilizing Ni3N@NiMoNx/NF as bifunctional electrodes for water splitting, an alkaline electrolyzer shows a low cell voltage of 1.68 V at 100 mA cm-2 with a superior durability of 80 h. Our work provides an experimental basis for advancing the rational design of efficient and stable bifunctional electrocatalysts for large-scale industrial water electrolysis applications.