A todorokite-type manganese oxide (T-MnO x ) with a 3×3 large tunnel structure was successfully synthesized by a hydrothermal method and employed as an active electrode material for a supercapacitor. The todorokite structure of the as-prepared manganese oxide was confirmed by X-ray diffraction (XRD), and the fragmented platelets were observed by scanning electron microscopy (SEM). The electrochemical performances of the as-prepared T-MnO x were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (CD) experiments and electrochemical impedance spectroscopy (EIS) in 1M Na2SO4 electrolyte. The specific capacitance of the T-MnO x electrode sharply increased during the initial 1000cycles and then continued improving gradually. It reached a maximum of 220Fg−1 at approximately the 4000th cycle at a scan rate of 2mVs−1, and further studies showed that the morphology and structure of the T-MnO x were maintained after 4000 cycles, indicating that an electrochemical activation process occurred during the initial thousands of cycles. The electrochemical activation mechanism is discussed in detail. The specific capacitance remained an almost constant value and decreased slightly upon prolonged cycling. After 23,000cycles, it decreased by 6% of the maximum specific capacitance. The experimental results demonstrate that T-MnO x is a promising candidate as an electrode material for supercapacitors.