Electrochemical water splitting into hydrogen and oxygen is a promising technology for sustainable energy storage. The development of earth-abundant transition metal phosphides (TMPs) to catalyze the hydrogen evolution reaction (HER) and TMP-derived oxy-hydroxides to catalyze the oxygen evolution reaction (OER) has recently drawn considerable attention. However, most monolithically integrated metal phosphide electrodes are prepared by laborious multi-step methods and their operational stability at high current densities has been rarely studied. Herein, we report a novel vapor-solid synthesis of single-crystalline cobalt phosphide nanowires (CoP NWs) on a porous Co foam and demonstrate their use in overall water splitting. The CoP NWs grown on the entire surface of the porous Co foam ligaments have a large aspect ratio, and hence are able to provide a large catalytically accessible surface over a given geometrical area. Comprehensive investigation shows that under the OER conditions CoP NWs are progressively and conformally converted to CoOOH through electrochemical oxidation/dephosphorization; the latter serving as an active species to catalyze the OER. The oxidized electrode shows exceptional electrocatalytic performance for the OER in 1.0 M KOH, delivering 100 mA cm at an overpotential ( ) of merely 300 mV and a small Tafel slope of 78 mV dec as well as excellent stability at various current densities. Meanwhile, the CoP NW electrode exhibits superior catalytic activity for the HER in the same electrolyte, affording -100 mA cm at = 244 mV and showing outstanding stability. An alkaline electrolyzer composed of two symmetrical CoP NW electrodes can deliver 10 and 100 mA cm at low cell voltages of 1.56 and 1.78 V, respectively. The CoP NW electrolyzer demonstrates exceptional long-term stability for overall water splitting, capable of working at 20 and 100 mA cm for 1000 h without obvious degradation.