<p>Revealing the surface structure evolution under working conditions is crucial for understanding the activity and stability of electrocatalysts. Platinum, the state-of-the-art electrocatalyst for hydrogen oxidation in fuel cells or hydrogen evolution in water splitting, is generally considered to be stable and little attention has been paid to its authentic structure during the hydrogen oxidation/evolution electrocatalysis. Herein, we disclose the in situ generation of surface hydrides on sub-2-nm Pt nanoparticles working under the electrode potentials of hydrogen electrocatalysis, showing the migration of surface-adsorbed hydrogen to interstitial hydrogen at the Pt subsurface. While weakening the metal-support interaction and inducing particle coalescence/detachment, the in situ generated surface hydride optimizes the binding energy of surface Pt with adsorbed hydrogen, leading to overall enhanced activities on hydrogen oxidation/evolution. The smaller the particle size, the lower the energy barrier for surface hydride formation, and consequently the more significant the activity enhancement, resulting in a pronounced particle size effect during long-term hydrogen electrocatalysis.</p>