Pursuit of robust electrocatalysts has been a persistent objective for hydrogen evolution from seawater. We present here an experimental realization of an efficient Co₃Mo₃C decorated carbon nanotube (Co₃Mo₃C/CNT) electrocatalyst with exceptionally high electrocatalytic activity and good durability through a simple thermal decomposition method for hydrogen evolution reaction (HER), yielding an onset overpotential as low as 42 mV and an exchange current density up to 0.415 mA cm⁻² in seawater. Notably, the resultant Co₃Mo₃C/CNT electrocatalyst displays a promising stability when suffering persistent operation over 26 h in seawater. In comparison with pristine Ni foam electrode, the Co₃Mo₃C/CNT can markedly reduce the overpotential for hydrogen evolution and therefore enhance HER kinetics.