Self-supported three-dimensional porous nickel phosphide (Ni–P) foam has been fabricated by exposing commercially available Ni foam in phosphorus vapor at an elevated temperature for a short period of time. The as-fabricated Ni–P foam consists of Ni2P skeletons covered with vertically aligned Ni5P4–NiP2 nanosheets. When used as a self-supported anode to catalyze the oxygen evolution reaction (OER), it exhibits exceptionally high catalytic current density (191.0 mA cm−2 at an overpotential of η = 0.35 V) and outstanding long-term stability and durability (affording 10 mA cm−2 at 1.45 V vs. RHE for 26 h without degradation). Scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry analyses show that during the galvanostatic OER electrolysis the surface Ni–P nanosheets are transformed to nickel oxide/hydroxide (NiO/Ni(OH)x), forming a Ni–P/NiO(Ni(OH)x) heterojunction on ligament surfaces of the porous Ni–P foam which would enhance the OER performance. The synergistic effect between Ni–P and NiO/Ni(OH)x is also confirmed by control experiments with self-supported NiO and Ni(OH)2 nanosheet electrodes. An alkaline electrolyzer has been built using two identical self-supported porous Ni–P foams as the anode and cathode, respectively. The electrolyzer exhibits superior electrolysis efficiency of 90.2% at 10 mA cm−2, and can maintain sufficiently high efficiency of 72.2% even at 100 mA cm−2. Moreover, the electrolyzer is substantially durable when working at 10 and 20 mA cm−2, splitting water constantly up to 1000 h with only a minor variation in the cell's voltage. The self-supported porous Ni–P foam holds substantial promise for use as both cathode and anode in industrial alkaline water electrolyzers.