The classic two-electron-reaction Volmer–Tafel–Heyrovský models in the cathodic hydrogen evolution reaction have been adopted in numerous studies for more than a hundred years without new advancement in terms of mechanistic study. In the Volmer-Heyrovský mechanism, the formation of monohydride (Pt-H) was considered. The Heyrovský step - electrochemical desorption of hydrogen involves simply a Pt-H, a proton (or a hydronium ion) and an electron in the step. However, it was not clearly described how the second hydrogen atom attaches to Pt-H and form hydrogen molecule. In this talk, discovery of a new dihydride mechanism in the hydrogen electrocatalysis on single atomic platinum is presented. We report that the HER occurs through the formation of an intermediate state of dihydride and demonstrate the possibility of the H-H bond formation between hydrides, which in turn generates the so-called Kubas-type intermediate. We present that mechanistic models built by the energy diagram of DFT calculation successfully simulated the experimental results which evidenced the presence of the new dihydride mechanism.