<p>Direct seawater electrolysis offers an attractive route for sustainable hydrogen production, yet its practical implementation remains constrained by the corrosive nature of chlorion (Cl⁻). Here, we reveal that trace amounts of Cl⁻, rather than being detrimental, can play a constructive role in directing the structural evolution of transition-metal catalysts. Guided by this insight, NiS₂ nanoparticles with controlled loading amounts on NiFe-MOF nanosheets (denoted as NiFe-MOF-S) are designed to enable fine regulation of Cl⁻ proximity to the active metal sites. This design promotes the in situ reconstruction of catalyst into a dense γ-NiFeOOH phase with short Ni–Fe bond lengths, which triggers a mechanistic shift from the lattice-oxygen-mediated pathway to oxide path mechanism. NiFe-MOF-S achieves high oxygen evolution performance in alkaline seawater (1 M KOH + seawater), requiring a much lower overpotential of 213 mV to reach 500 mA cm⁻² in alkaline seawater than that in alkaline water (1 M KOH) (268 mV). Furthermore, the electrocatalyst demonstrates high durability with a degradation rate of only 1.4 μV h⁻¹ for over 7000 hours at 1.0 A cm⁻² in alkaline seawater. A kilowatt-level alkaline seawater electrolyzer equipped with NiFe-MOF-S/NF operates reliably for over 1500 hours at industrially relevant conditions.</p>