The metal-lightweighted electrocatalysts for water splitting are highly desired for sustainable and economic hydrogen energy deployments, but challengeable. In this work, a low-content Ni-functionalized approach triggers the high capability of black phosphorene (BP) with hydrogen and oxygen evolution reaction (HER/OER) bifunctionality. Through a facile in situ electro-exfoliation route, the ionized Ni sites are covalently functionalized in BP nanosheets with electron redistribution and controllable metal contents. It is found that the as-fabricated Ni-BP electrocatalysts can drive the water splitting with much enhanced HER and OER activities. In 1.0 M KOH electrolyte, the optimized 1.5 wt% Ni-functionalized BP nanosheets have readily achieved low overpotentials of 136 mV for HER and 230 mV for OER at 10 mA cm-2. Moreover, the covalently bonding between Ni and P has also strengthened the catalytic stability of the Ni-functionalized BP electrocatalyst, stably delivering the overall water splitting for 50 h at 20 mA cm-2. Theoretical calculations have revealed that Ni–P covalent binding can regulate the electronic structure and optimize the reaction energy barrier to improve the catalytic activity effectively. This work confirms that Ni-functionalized BP is a suitable candidate for electrocatalytic overall water splitting, and provides effective strategies for constructing metal-lightweighted economic electrocatalysts.