The dimensional confinement endows ultrathin nanosheets with unique physical and chemical properties, which have been widely studied for the purpose of developing active electrocatalysts for water splitting. Ultrathin nanosheets are generally synthesized by chemical vapor deposition, exfoliation, or surfactant-assisted synthesis, which either require special equipment and reaction conditions, or is limited by the low yields and the difficulty in controlling the lateral size and structure of the nanosheets. In addition, achieving a high loading of ultrathin nanosheets on the electrodes without compromising their catalytic activity still remains a challenge. Herein, we report a simple electrodeposition method for preparing Co₃O₄ and Co(OH)₂ ultrathin nanosheet arrays (UNA) without using templates or surfactants. The obtained arrays exhibit high activity for oxygen and hydrogen evolution reactions, in both alkaline and neutral media. The electrolyzer based on Co₃O₄ and Co(OH)₂ UNA shows superior activity and stability than that based on IrO₂ and Pt/C, which demonstrates the potential of the present electrodeposition method for developing active and stable electrocatalysts for water splitting.