First, under acidic conditions, analysis of operando spectroscopic monitoring reveals that the electrochemical preactivation process of o-CoSe2 promotes surface corrosion with the removal of oxidized oxyanion species, and then the reconstructed o-CoSe2 undergoes an electrochemically driven structural optimization into the generation of disordered catalytically active [CoSe6] motifs for the H2 release. In contrast, such surface corrosion along with electrochemically driven restructuring into active species was not observed in c-CoSe2 and c-S-CoSe2, which were demonstrated as stable catalysts throughout the entire acidic HER. Combinations of the aforementioned operando spectroscopic characterizations and DFT calculations elucidate that the acidic HER of o-CoSe2 proceeds with the adsorption/desorption of H* intermediates at the Se sites within the defective [CoSe6] motifs, which features a lower energy barrier compared to that of c-CoSe2. Partial S incorporation in CoSe2 not only enables a newly induced Co d-S p orbital hybridization with facilitated HER kinetics but also stabilizes the active S sites during the reaction process. As a result of the tuned HER kinetics, the as-prepared c-S-CoSe2 catalysts demonstrated exceptional acidic HER performance with a low overpotential of -88 mV at -10 mA/cm 2 and robust electrochemical stability at 20 m/cm 2 for over 300 h.