AbstractAliovalent doping is widely adopted to tune the electronic structure of transition‐metal oxides for design of low‐cost, active electrocatalysts. Here, using single‐crystalline thin films as model electrocatalysts, the structure‐activity relationship of Fe states doping in perovskite LaNiO₃ for oxygen evolution reaction (OER) is studied. Fe⁴⁺ state is found to be crucial for enhancing the OER activity of LaNiO₃, dramatically increasing the activity by six times, while Fe³⁺ has negligible effect. Spectroscopic studies and DFT calculations indicate Fe⁴⁺ states enhance the degree of Ni/Fe 3d and O 2p hybridization, and meanwhile produce down‐shift of the unoccupied density of states towards lower energies. Such electronic features reduce the energy barrier for interfacial electron transfer for water oxidization by 0.2 eV. Further theoretical calculations and H/D isotope experiments reveal the electronic states associated with Fe⁴⁺‐O²⁻‐Ni³⁺ configuration accelerate the deprotonation of *OH to *O (rate‐determining step), and thus facilitate fast OER kinetics.