Hydrous manganese dioxide nanostructures were synthesized via a catalytic oxidation reaction mechanism at mild temperatures. It was found that the morphology of the manganese dioxide nanostructures was significantly influenced by the pH of the reaction system. With increasing pH the morphology of manganese dioxide nanostructures changed from urchin-like structures to nanobelts. The capacitive performance was investigated by using cycle voltammetry and galvanostatic charge/discharge techniques. Hydrous manganese dioxide nanostructures obtained from a basic solution exhibited a capacitance of 262 F g(-1) at a current density of 250 mA g(-1) and a capacitive retention of 75% after 1200 cycles, suggesting that this is a promising electrode material for supercapacitors. The high specific capacitance is attributed to the hydrous nature coupled with a high surface area (181 m(2) g(-1)) of the manganese dioxide nanostructure.