The development of efficient, low-cost, stable, non-noble-metal electrocatalysts for water splitting, particularly those that can catalyze both the hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode, is a challenge. We have developed a facile method for synthesizing CoSe₂ nanoparticles uniformly anchored on carbon fiber paper (CoSe₂/CF) via pyrolysis and selenization of in situ grown zeolitic imidazolate framework-67 (ZIF-67). CoSe₂/CF shows high and stable catalytic activity in both the HER and OER in alkaline solution. At a low cell potential, i.e., 1.63 V, a water electrolyzer equipped with two CoSe₂/CF electrodes gave a water-splitting current of 10 mA·cm^-2. At a current of 20 mA·cm^-2, it can operate without degradation for 30 h. This study not only offers a cost-effective solution for water splitting but also provides a new strategy for developing various catalytic nanostructures by changing the metal–organic framework precursors.