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Abstract Atomic‐scale engineering of chromite spinels featuring redox‐active tetrahedral A‐sites and strong Cr–O covalency offers a promising route to superior platinum‐group‐metal‐free oxygen evolution reaction (OER) catalysts. However, comprehensive studies addressing how cation substitution influences surface chemistry and governs OER activity and durability in chromite spinels remain limited. Here, a systematic investigation of the multicationic chromite series Ni_xFe_yCr_3−x−yO₄is presented, identifying composition‐dependent Lewis acidity as a descriptor of superior OER performance. It is further demonstrated that tuning surface acidity directly controls dynamic reconstruction processes and lattice‐oxygen participation during spinel‐based electrocatalysis. Following activation, the optimized Ni₀.₈Fe₀.₃Cr₁.₉O₄catalyst delivers a current density of 10 mA cm⁻²at an overpotential of 235 mV, surpassing RuO₂, with excellent long‐term stability. Integrating microscopic and spectroscopic analysis with operando impedance spectroscopy, it shows that activation generates an oxyhydroxide overlayer and reveals a previously unrecognized link between surface Lewis acidity and the growth kinetics and activity of these shells. Density functional theory calculations indicate that Fe incorporation at octahedral sites raises the O 2p‐band center and lowers oxygen‐vacancy formation energy, promoting lattice‐oxygen activation and triggering reconstruction, yielding enhanced OER. This work integrates cation‐driven surface‐acidity modulation, acidity‐governed reconstruction, and OER activity enhancement into a unified predictive framework for designing earth‐abundant spinel‐based catalysts.