One-dimensional (1D) nanoarrays are beneficial for electrochemical reactions, including water splitting, due to their directional electron transport through the array, large effective surface area, and porosity. Fabrication of such array without a template is, however, extremely challenging. Herein, an ion-exchange approach is employed to fabricate a 1D cobalt sulfide nanorod array (1D-CoS) from Co(3)O(4)on stainless steel. The 1D-CoS electrode works as a bifunctional electrocatalyst, attaining the current density of 10 mA cm(-2)for hydrogen evolution reaction and oxygen evolution reaction at a low overpotential of 159 and 280 mV, respectively in a 1 M KOH solution. A full electrolyzer with the same two 1D-CoS electrodes assembly demonstrates a low cell voltage of 1.75 V at 10 mA cm(-2)and this performance value is well maintained for 20 hours. The efficient catalytic performance can be due to the controlled CoS phase with a unique 1D nanoarchitecture, which allows facile electrolyte diffusion and charge transfer between electrode/electrolyte interface.