<p>Direct electrolysis of seawater for hydrogen production offers a vital route to circumvent freshwater limitations and advance a sustainable hydrogen economy. Herein, we present a novel catalyst design strategy based on the co-doping of Ru atoms with strong chloride affinity and Cr atoms serving as Lewis acid centers into a nickel phosphide matrix (RuCr-Ni₃P). This material is a highly active anode for direct alkaline seawater electrolysis, offering outstanding oxygen evolution reaction (OER) selectivity and operational longevity exceeding 4000 h under industrially relevant conditions. Mechanistic investigations reveal that chloride ions are not merely detrimental impurities, but are selectively captured by Ru sites to form a dynamic Ru–Cl coordination motif. This interaction electronically modulates adjacent Ni active centers, facilitating the generation of high-valent Ni^&gt;3+ species and switching the dominant OER pathway from the lattice oxygen mechanism (LOM) to the more efficient adsorbate evolution mechanism (AEM). Simultaneously, Cr sites promote the formation of Cr–OH species, which cooperatively create a localized alkaline microenvironment favorable for OER on the high-valent Ni centers. This dual-site synergistic mechanism, wherein Ru sites regulate chloride and Ni/Cr sites drive catalysis, effectively transforms Cl⁻ from a performance-limiting species into a chemical switch that concurrently enhances both activity and stability.</p>