The advancement of cost-effective electrocatalysts with high activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in water splitting is chief to renewable energy technology. In this work, an Fe-doped NiSe (a mixture of hexagonal NiSe and orthorhombic NiSe) nanorod/nanosheet hierarchical array on Ni foam was synthesized through a one-step solvothermal reaction, which exhibits high and stable catalytic activity for both the OER and HER in alkaline solution. The optimal Fe_7.4%-NiSe only requires a small overpotential of 231 mV to drive 50 mA cm⁻², and can provide a large current density of 500 mA cm⁻² at an overpotential of 269 mV, and 1000 mA cm⁻² at 291 mV for the OER. Moreover, it can afford 10 mA cm⁻² at an overpotential of 163 mV, and 500 mA cm⁻² at 296 mV toward the HER. A water electrolyzer with two Fe_7.4%-NiSe electrodes only needs a low cell potential of 1.585 V to reach 10 mA cm⁻², and 1.74 V to reach 100 mA cm⁻² with great stability. The rich active sites and high conductivity of the Fe-doped NiSe nanorod/nanosheet hierarchical array contribute to its high catalytic performance. This work develops an alternative and simple synthetic approach for bimetal selenides with a hierarchical structure and provides an attractive low-cost electrocatalyst toward overall water splitting applications.