Cobalt selenide has been proposed to be an effective low‐cost electrocatalyst toward the oxygen evolution reaction (OER) due to its well‐suited electronic configuration. However, pure cobalt selenide has by far still exhibited catalytic activity far below what is expected. Herein, this paper for the first time reports the synthesis of new monoclinic Co3Se4 thin nanowires on cobalt foam (CF) via a facile one‐pot hydrothermal process using selenourea. When used to catalyze the OER in basic solution, the conditioned monolithic self‐supported Co3Se4/CF electrode shows an exceptionally high catalytic current of 397 mA cm−2 at a low overpotential (η) of 320 mV, a small Tafel slope of 44 mV dec−1, a turnover frequency of 6.44 × 10−2 s−1 at η = 320 mV, and excellent electrocatalytic stability at various current densities. Furthermore, an electrolyzer is assembled using two symmetrical Co3Se4/CF electrodes as anode and cathode, which can deliver 10 and 20 mA cm−2 at low cell voltages of 1.59 and 1.63 V, respectively. More significantly, the electrolyzer can operate at 10 mA cm−2 over 3500 h and at 100 mA cm−2 for at least 2000 h without noticeable degradation, showing extraordinary operational stability. Thin Co3Se4 nanowires are grown on porous Co foam (CF) via hydrothermal selenization, forming an integrated Co3Se4/CF electrode, which exhibits outstanding catalytic performance for oxygen evolution with a high current of 397 mA cm‐2 at an overpotential of 320 mV. An electrolyzer comprising two symmetrical Co3Se4/CF can operate at 10 mA cm−2 under 1.59 V over 3500 h without degradation.