To date, nanostructures of 3d-group transition metal (i.e., Fe, Co, Ni, etc.) derivatives show the highest electrocatalytic performance among non-noble-metal electrocatalysts for water splitting in acidic electrolyte. However, the poor electrochemical conductivity (similar to 10(-4) S/cm) of nanostructures restricts practical application for overall electrocatalytic activity. Herein, continuously networked nanostructures of phase-tuned nickel sulfide foams for efficient water splitting electrocatalysts in both acidic and alkaline electrolytes are reported. Because continuously networked nanostructures of nickel sulfide foams possess an integral structure, they exhibit high electrochemical conductivity (similar to 1 S/cm), which eases adsorption/desorption of H+ and OH- ions for efficient overall water splitting. By tuning the stoichiometry of sulfur, four different phases of continuously networked nanostructures of nickel sulfides (alpha NiS, beta NiS, Ni3S2, and Ni7S6) foams are formed by facile phase transformation of nickel. Among them, the Ni7S6 foam (Ni7S6-F) possesses superior electrocatalytic activity with extremely low overpotential of 70 mV (for hydrogen evolution reaction) and 1.37 V (for oxygen evolution reaction) at 10 mA/cm(2) in acidic and alkaline medium, respectively, which is close to noble-metal-based electrocatalysts. As a result, this work demonstrates a facile synthesis route to optimize nickel sulfide electrocatalysts through phase-tuning and continuous networking for overall water splitting and would be applicable on other nanostructured electrocatalysts to improve their electrocatalytic activity for practical applications in future energy devices.