Discovering novel oxygen evolution reaction (OER) (pre)catalysts with exceptional catalytic activity and long‐term stability is pivotal for advancing decarbonization technologies. In this study, we present the ternary Ba8Ni6Ge40 phase with an open clathrate structure exhibiting remarkable performance in alkaline OER. When integrated into an alkaline water electrolyzer, this clathrate precatalyst achieves high stability under a sustained current density of ∼550 mA cm−2 for 10 days. By combining in situ Raman spectroscopy, quasi in situ X‐ray absorption spectroscopy, and (micro)structural characterizations, we elucidate the complete electrochemical transformation of Ba8Ni6Ge40 (~90 weight% leaching) forming ultrathin nanosheets composed of a porous and defective NiOOH nanostructure with maximized accessible active site exposure. Notably, a reversible phase transition mainly between Ni(OH)2 and NiOOH has also been established in the electrochemical redox process. Meanwhile, the successful application of the model Ba8Ni6Ge40 precatalyst represents a promising new class of functional inorganic materials for water electrolysis.
