Alkaline seawater electrolysis is promising for large-scale production of green hydrogen but the chlorine evolution reaction (CER) causes severe anode’s corrosion under high current densities. This work described the use of a sulfur-doped NiFe layered double hydroxide nanoarray on Ni foam (S-NiFe LDH/NF) synthesized through a two-step hydrothermal process as a durable catalyst for alkaline seawater oxidation. In 1 M KOH + seawater, the S-NiFe LDH/NF anode needs a low overpotential of 345 mV to afford a current density of 1000 mA·cm⁻² and operates stably over 800 h. Sulfate species generated on the catalyst surface, which is evidenced by in situ Raman spectroscopy analysis, electrostatically repel Cl⁻ and thus inhibits the CER. Furthermore, the two-electrode system using S-NiFe LDH/NF and Pt/C/NF as the anode and cathode, respectively, requires a cell voltage of 1.90 V to achieve a current density of 100 mA·cm⁻² and maintains stable operation for 1000 h at 500 mA·cm⁻² in alkaline seawater.