Developing earth-abundant catalysts for overall water splitting is important to sustainable energy conversion technologies. However, selectively exposing active surface and judiciously optimizing nanostructure remain challenging for high-performance electrocatalysts. Herein, (003)-facet-exposed Ni3S2 nanoporous thin films (NTFs) on nickel foil are synthesized through anodization and vapor-sulfurization strategies. The Ni3S2 NTFs exhibit low onset overpotentials of 48 and 152 mV, and require overpotentials of 135 and 175 mV to achieve a benchmark of 10 mA cm(-2) towards alkaline hydrogen and oxygen evolution, respectively. Remarkably, an electrolyzer assembled by Ni3S2 NTFs as both anode and cathode can deliver a water-splitting current density of 10 mA cm(-2) at 1.611 V. Experimental and theoretical results indicate that the controllable hollow nanoporous spheres and low-coordinated Ni-3-triangles on Ni3S2(003) facet synergistically contribute to the electrocatalytic activity.