We report a simple one-step hydrothermal method to fabricate cobalt (Co)-doped vanadium diselenide (VSe₂) nanosheets on a carbon cloth for the hydrogen evolution reaction (HER) in acid solution. We find that the Co-doped VSe₂ shows significantly enhanced catalytic activity compared with the pristine VSe₂, including a low overpotential of 230 mV at 10 mA/cm², a small Tafel slope of 63.4 mV/dec, and high stability. Our density functional theory calculations show that Co dopants dramatically reduce the Gibbs free energy for hydrogen adsorption (ΔG_H), leading to improved catalytic activity for hydrogen evolution. Moreover, the Co doping promotes electron transfer and HER kinetics. Our results provide a valuable route to improve the catalytic performance of metal diselenide in HER.