Direct seawater electrolysis offers a promising approach for large-scale hydrogen production, but it is challenged by harmful chlorine chemistry and high energy costs. Sulfur oxidation reaction (SOR) as an alternative to the slow oxygen evolution reaction (OER) is a low-energy-consuming seawater hydrogen production technology that can simultaneously degrade industrial sulfur-containing wastewater. However, the limited availability of efficient and stable catalysts has hindered its development. In this work, chlorine-free seawater splitting coupled with a crystalline/amorphous strategy to promote electrocatalytic SOR for energy-efficient hydrogen production is reported. We propose a bifunctional amorphous FeNi₂P nanosheet embedded with crystalline nanoparticles (c/a-FeNi₂P) electrocatalyst, which exhibits excellent SOR and hydrogen evolution reaction (HER) performance. In situ Raman spectroscopy and density functional theory calculations reveal that the unique crystalline/amorphous strategy optimizes the adsorption of sulfide and polysulfide ions and the efficient desorption of S₈, thereby enhancing catalytic activity and stability. c/a-FeNi₂P enables efficient SOR-assisted seawater electrolysis. In the SOR–HER system, c/a-FeNi₂P demonstrates an ultralow voltage of 0.548 V at 100 mA·cm⁻² and stable operation for 200 h at 170 mA·cm⁻², showcasing remarkable durability. This hybrid seawater electrolyzer provides a promising method for hydrogen production from seawater electrolysis, demonstrating great potential for energy conservation and environmental remediation.