Developing efficient and durable oxygen evolution reaction (OER) catalysts holds great promise for green hydrogen production via seawater electrolysis, but remains a challenge. Herein, we report a Co-doped Ni₃S₂ nanosheet array on Ni foam (Co-Ni₃S₂/NF) as a high-efficiency OER electrocatalyst for seawater. In alkaline conditions, Co-Ni₃S₂/NF requires an overpotential of only 368 mV to drive 100 mA·cm^–2, superior to Ni₃S₂/NF (385 mV). Besides, it exhibits at least 50-h continuous electrolysis.