<p>Electrocatalytic reduction of carbon dioxide and water oxidation are promising technologies to mitigate environmental problems. A critical bottleneck, however, is the significant energy loss that arises from the anodic oxygen evolution reaction (OER) with its sluggish kinetics and reliance on scarce noble metals. It is therefore essential to develop earth-abundant and efficient OER catalysts. Here we report the reactive diiron electrocatalyst [Fe₂(µ-O)(µ-OH)(L₁)₂], where L₁ is a nitrogen-based ligand, which exhibits an outstanding performance—achieving a turnover frequency of 20.2 s⁻¹ at 1.580 V and a low overpotential of 184 mV at a current density of 10 mA cm⁻²—and exceptional stability over 1,000 h. This diiron electrocatalyst is formed via a spin crossover-driven dimerization mechanism, where the resulting diiron atomic configuration promotes strong metal–ligand covalency and facilitates the formation of key intermediates that are essential for efficient OER catalysis. Our findings offer a promising strategy for the design of high-performance catalysts for water oxidation and sustainable electrocatalysis.</p>