Development of efficient non-precious catalysts for seawater electrolysis is of great significance but challenging due to the sluggish kinetics of oxygen evolution reaction (OER) and the impairment of chlorine electrochemistry at anode. Herein, we report a heterostructure of Ni₃S₂ nanoarray with secondary Fe-Ni(OH)₂ lamellar edges that exposes abundant active sites towards seawater oxidation. The resultant Fe-Ni(OH)₂/Ni₃S₂ nanoarray works directly as a free-standing anodic electrode in alkaline artificial seawater. It only requires an overpotential of 269 mV to afford a current density of 10 mA·cm⁻² and the Tafel slope is as low as 46 mV·dec⁻¹. The 27-hour chronopotentiometry operated at high current density of 100 mA·cm⁻² shows negligible deterioration, suggesting good stability of the Fe-Ni(OH)₂/Ni₃S₂@NF electrode. Faraday efficiency for oxygen evolution is up to ~ 95%, revealing decent selectivity of the catalyst in saline water. Such desirable catalytic performance could be benefitted from the introduction of Fe activator and the heterostructure that offers massive active and selective sites. The density functional theory (DFT) calculations indicate that the OER has lower theoretical overpotential than Cl₂ evolution reaction in Fe sites, which is contrary to that of Ni sites. The experimental and theoretical study provides a strong support for the rational design of high-performance Fe-based electrodes for industrial seawater electrolysis.