 A MnO2 nanoparticle/three-dimensional N-doped reduced graphene oxide (MnO2/3D-NRGO) was prepared through a hydrothermal method with a subsequent ultrasound treatment in a KMnO4 aqueous solution. Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope, and transmission electron microscope analyses indicate that the MnO2 nanoparticles tightly anchored onto the N-doped graphene sheets in the MnO2/3D-NRGO. Considering the increased bonding force and the 3D porous graphene with 3D interpenetrating microstructures, large accessible surface areas, and interconnected conductive networks, as a supercapacitor electrode material, MnO2/3D-NRGO showed specific capacitance of 349 F g−1 at 0.5 A g−1 in a 1.0 M Na2SO4 electrolyte solution and superior cyclic stability (106% of initial capacitance after 20000 cycles). The asymmetric supercapacitor fabricated with the MnO2/3D-NRGO and 3D-NRGO as the positive and negative electrode material, respectively, exhibited a high energy density (35.22 Wh kg−1 at 200 W kg−1), suggesting that MnO2/3D-NRGO is a promising electrode material for supercapacitors.