Small-sized Co-doped Ni2P nanocrystals (Co-Ni2P) display excellent electrocatalytic activity and superior chlorine resistance in alkaline seawater media. The Co-Ni2P‖Co-Ni2P electrolyzer affords a low cell voltage of 1.61 V at 10 mA cm-2 and retains remarkable stability, showing promising prospects for alkaline seawater electrolysis.