To realize nonprecious-metal catalysts with practical applicability for the hydrogen evolution reaction (HER), improved corrosion resistance and catalytic activity are required. In this study, composition-controlled Co-Cu alloys were fabricated by electrodeposition for use as HER catalysts in proton exchange membrane water electrolyzers (PEMWEs). As the Cu content in the alloy increased, the morphology changed from needle-shaped particles to small round particles. Furthermore, a phase transition from a hexagonal close-packed structure to a face-centered cubic structure occurs because the latter structure is stabilized by adding Cu to Co. The optimum catalyst composition for the HER was found to be Co59Cu41, which had an overpotential of 342 mV at -10 mA cm(-2). This catalyst exhibited excellent durability, showing a potential reduction of approximately 100 mV over 12 hours under a constant current density. This superior performance was attributed to the increase in the electrochemical surface area resulting from the addition of Cu, as confirmed by electrochemical double layer capacitance measurements, in addition to a counterbalance between the hydrogen adsorption energies of Co and Cu. Finally, the application of the Co-Cu alloy catalyst as a cathode catalyst in a PEMWE resulted in excellent performance of 1.2 A cm(-2) at 2.0 V-cell.