Direct seawater electrolysis offers a sustainable route to hydrogen, but catalyst corrosion and degradation of polymer membranes under alkaline, chloride-rich conditions remain key limitations. We report a zero-gap alkaline seawater electrolyzer pairing a Ru (8 mol %)-modulated CoFe₂O₄ (Ru-CFO) anode with a zirconia-based porous diaphragm, which circumvents polymer degradation. Ru-CFO delivers oxygen evolution reaction (OER) overpotentials of 351 and 428 mV at 100 mA cm^–2 in 1 M KOH and 1 M KOH seawater, outperforming RuO₂. Rotating ring-disk electrode and in situ Raman measurements confirm selective OER without chloride adsorption or chlorine evolution. Density functional theory reveals that Ru incorporation expands the Fe-dominated pathway into a dual active motif engaging individual Fe centers and cooperative Co–Fe bridges. In a single cell, Ru-CFO reaches 2.69 A cm^–2 at 2.5 V in 6 M KOH seawater, comparable to state-of-the-art anion exchange membrane water electrolyzers, with an activity-stability factor 4.9 times that of RuO₂.​