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Due to the robust oxidation conditions in strong acid oxygen evolution reaction (OER), developing an OER electrocatalyst with high efficiency remains challenging in polymer electrolyte membrane (PEM) water electrolyzer. Recent theoretical research suggested that reducing the coordination number of Ir−O is feasible to reduce the energy barrier of the rate‐determination step, potentially accelerating the OER. Inspired by this, we experimentally verified the Ir−O coordination number's role at model catalysts, then synthesized low‐coordinated IrOx nanoparticles toward a durable PEM water electrolyzer. We first conducted model studies on commercial rutile‐IrO2 using plasma‐based defect engineering. The combined in situ X‐ray absorption spectroscopy (XAS) analysis and computational studies clarify why the decreased coordination numbers increase catalytic activity. Next, under the model studies' guidelines, we explored a low‐coordinated Ir‐based catalyst with a lower overpotential of 231 mV@10 mA cm−2 accompanied by long durability (100 h) in an acidic OER. Finally, the assembled PEM water electrolyzer delivers a low voltage (1.72 V@1 A cm−2) as well as excellent stability exceeding 1200 h (@1 A cm−2) without obvious decay. This work provides a unique insight into the role of coordination numbers, paving the way for designing Ir‐based catalysts for PEM water electrolyzers. [ABSTRACT FROM AUTHOR]