The development of nonprecious, highly active and robust bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions via water splitting is of primary significance, but still remains challenging. Herein, we develop a three-dimension (3D) heterostructured catalyst (Ni0.82Co0.18O@C/NF) by coupling of ultrathin Co-doped NiO (Ni0.82Co0.18O) nanosheets and carbon on nickel foam (NF) with synergism of geometric engineering and electronic modulation. Benefiting from the unique 3D configuration, highly exposed active sites and the synergistic effect of the active Ni0.82Co0.18O and the carbon, the catalyst exhibits exceptional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance with low overpotentials and Tafel slopes and excellent stability in alkaline media. Specifically, the NNi0.82Co0.18O@C/NF employed as catalyst for both anode and cathode enables an alkaline electrolyzer to achieve a current density of 10 mA cm(-2) by a cell voltage of only 1.42 V, which is not only much lower than that of the integrated performance of the commercial noble Pt/C and IrO2/C catalyst couple (similar to 1.61 V), but also superior to most reported results to date. Our finding may provide a new opportunity to design advanced bifunctional catalysts toward practical overall water splitting.