Studies of the electrochemical behavior of K0.27MnO2·0.6H2O in K2SO4 show the reversible intercalation/deintercalation of K+-ions in the lattice. An asymmetric supercapacitor activated carbon (AC)/0.5moll−1 K2SO4/K0.27MnO2·0.6H2O was assembled and tested successfully. It shows an energy density of 25.3Whkg−1 at a power density of 140Wkg−1; at the same time it keeps a very good rate behavior with an energy density of 17.6Whkg−1 at a power density of 2kWkg−1 based on the total mass of the active electrode materials, which is higher than that of AC/0.5moll−1 Li2SO4/LiMn2O4. In addition, this asymmetric supercapacitor shows excellent cycling behavior without the need to remove oxygen from the electrolyte solution. This can be ascribed in part to the stability of the lamellar structure of K0.27MnO2·0.6H2O. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.