Rational design of highly efficient and durable electrocatalysts with low cost to replace noble-metal based catalysts for seawater electrolysis is extremely desirable, but challenging. In this work, we demonstrate a rapid electrodeposition method by growing P-Ni₄Mo on the surface of the copper foam (CF) substrate to synthesize an efficient seawater electrolysis catalyst (P-Ni₄Mo/CF). The catalyst exhibited considerable HER performance and stability in alkaline seawater, with the overpotential as low as 260 mV at a current density of 100 mA cm^-2. The P-Ni₄Mo/CF is capable of achieving 1.0 A cm^-2 with an overpotential of 551 mV, which is slightly worse than that of the Pt/C catalyst (453 mV). Moreover, P-Ni₄Mo/CF demonstrates robust durability, with almost no activity loss after the durability test for more than 200 h. This work not only reports a new catalyst for seawater electrolysis, but also presents a strategy for the performance enhancement of seawater electrolysis.