Catalytic water splitting driven by renewable electricity offers a promising strategy to produce molecular hydrogen, but its efficiency is severely restricted by the sluggish kinetics of the anodic water oxidation reaction. Amorphous catalysts are reported to show better activities of water oxidation than their crystalline counterparts, but little is known about the underlying origin, which retards the development of high-performance amorphous oxygen evolution reaction catalysts. Herein, on the basis of cyclic voltammetry, electrochemical impedance spectroscopy, isotope labeling, and in situ X-ray absorption spectroscopy studies, we demonstrate that an amorphous catalyst can be electrochemically activated to expose active sites in the bulk thanks to the short-range order of the amorphous structure, which greatly increases the number of active sites and thus improves the electrocatalytic activity of the amorphous catalyst in water oxidation.