AbstractSeawater electrolysis offers a renewable, scalable, and economic means for green hydrogen production. However, anode corrosion by Cl^- pose great challenges for its commercialization. Herein, different from conventional catalysts designed to repel Cl^- adsorption, we develop an atomic Ir catalyst on cobalt iron layered double hydroxide (Ir/CoFe-LDH) to tailor Cl^- adsorption and modulate the electronic structure of the Ir active center, thereby establishing a unique Ir-OH/Cl coordination for alkaline seawater electrolysis. Operando characterizations and theoretical calculations unveil the pivotal role of this coordination state to lower OER activation energy by a factor of 1.93. The Ir/CoFe-LDH exhibits a remarkable oxygen evolution reaction activity (202 mV overpotential and TOF = 7.46 O₂ s⁻¹) in 6 M NaOH+2.8 M NaCl, superior over Cl^--free 6 M NaOH electrolyte (236 mV overpotential and TOF = 1.05 O₂ s⁻¹), with 100% catalytic selectivity and stability at high current densities (400-800 mA cm⁻²) for more than 1,000 h.