An nanoporous NiMnFeMo alloy is exploited for over water splitting in basic solutions. The free-standing 3D nanoporous electrode exhibits both outstanding HER and OER catalytic activities even at high current density in basic electrolyte, only 1.54V for full water splitting at 10 mA cm−2. Display omitted •A free-standing nanoporous NiMnFeMo alloy with ultrahigh catalytic activity is synthesized.•The electrode can stably deliver 1000 mA cm−2 at only 290 mV for HER and 570 mV for OER.•The synergetic reaction of Ni with Mn, Fe and Mo results in good HER preformance. Although significant progresses have been achieved recently in developing efficient catalysts for electrochemical water splitting, high performance catalysts toward hydrogen evolution and oxygen evolution in alkaline electrolyte at high current density (≧1000 mA cm−2) have been seldom realized. Herein, we report a flexible and free-standing nanoporous NiMnFeMo alloy (np-NiMnFeMo) with ultrahigh catalytic activity as both anode and cathode even at high current density. The nanoporous NiMnFeMo alloy can deliver as high as 1000 mA cm−2 at an overpotential of only 290 mV for hydrogen evolution reaction and 570 mV for oxygen evolution reaction. DFT calculations indicate that the ultrahigh HER activity of the catalyst is originated from the synergetic effect of the solid solution elements, where Ni atoms act as water dissociation center in the np-NiMnFeMo and the other metals (Mn, Fe and Mo) regulate the electronic structure and provide superior adsorption properties towards hydrogen. More importantly, the electrolyzer, assembled using the np-alloys as both cathode and anode for full water splitting, shows excellent stability.