In this work, a rapid hydrothermal method has been developed to synthesize hierarchical porous nanostructures assembled from ultrathin birnessite-type MnO2 nanosheets without using any template and surfactant. The microstructure, morphology, and thermal stability of the products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption–desorption isotherms, and thermal gravimetric analysis. The electrochemical properties of the as-prepared MnO2 as an electrode material for supercapacitor were investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements in 1M Na2SO4 electrolyte, and a specific capacitance of 269Fg−1 was achieved at a current density of 0.3Ag−1. The specific capacitance retention and coulombic efficiency were 94% and 98% respectively after 2000 cycles at 1Ag−1. Furthermore, asymmetric supercapacitor was also fabricated with MnO2 as the positive electrode and activated carbon as the negative electrode in 1M Na2SO4 electrolyte with the cell voltage to 2V.