Three-dimensional (3D) nitrogen-doped carbon materials with a hierarchically porous structure are prepared by the introduction of nitrogen-doped carbon spheres (NCS) into the inter-sheet spaces of graphitic carbon nitride nanosheets (g-CN). The as-prepared graphitic carbon nitride/nitrogen-doped carbon sphere (g-CN/NCS) composites present a high nitrogen doping level, a unique hierarchically porous structure, and a high specific surface area of 448 m2 g-1. Such particular features make the g-CN/NCS composite an ideal material for supercapacitor electrodes, which could deliver a large specific capacitance of 403.6 F g-1 at 0.1 A g-1, an excellent rate capability of 220 F g-1 at 10 A g-1, and a high cycling stability with almost 100% capacitance retention after 5000 cycles at 20 A g-1. Furthermore, the g-CN/NCS electrode-based symmetric supercapacitors exhibit a decent energy density of 6.75 W h kg-1 at a power density of 1000 W kg-1. The enhanced performances are mainly attributed to the high nitrogen doping level and the hierarchically porous structure of the 3D structured g-CN/NCS composites, which provide an efficient pathway for transporting ions and electrons, and endow more active sites for electrochemical energy storage.