Herein, we report a rapid and simultaneous synthesis of manganese oxide (MnO2) nanoparticles deposited on porous graphene foam by microwave-assisted method. The microwave irradiation enables simultaneously reduction and porous structure of graphene oxide and deposition of MnO2. As-synthesized MnO2/graphene nanocomposites are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The resultant MnO2/graphene nanocomposite electrodes exhibit as high as specific capacitance of 372Fg−1 at 0.5Ag−1, high rate capability (82% retention at 5Ag−1), and good long-term stability (98.5% retention during 3000cycles). This synthetic method could be applicable to the mass production of porous nanocomposites as energy storage materials for supercapacitors and lithium-ion batteries.