Developing cost-effective and stable bifunctional catalysts for overall water splitting is of enormous importance for the realization of sustainable clean-energy technologies. Herein, a monometallic NiO-Ni₃S₂ heteronanosheets supported on Ni foam are rationally constructed as bifunctional electrocatalysts for overall water splitting. On account of the unique electronic property of the abundant interface and its 3D interconnected integration structure, NiO-Ni₃S₂ heteronanosheets exhibit high electrocatalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Utilizing NiO-Ni₃S₂/NF as both the anode and the cathode for overall water splitting, a current density of 10 mA cm⁻² is achieved at a cell voltage of as low as 1.57 V with excellent stability, exceeding that of the integrated performance of Pt/C and RuO₂. DFT calculations further reveal that key role of the established interfaces between Ni₃S₂ and NiO in modulating the chemisorption of hydrogen and oxygen-containing intermediates, and consequently improving the overall electrochemical water-splitting activity.