Manganese dioxide (MnO2) nanostructures have been widely used as the supercapacitor electrode. However, their charge storage mechanism has not yet been fully understood. Herein, the charge storage mechanism of layered MnO2 nanosheets has been investigated. It was found that they store charges via three processes: electrochemical double layer capacitance (EDLC), surface redox reaction, and intercalation/de-intercalation. The EDLC of the layered MnO2 is equal to 71.6Fg−1 at all scan rates used (10–100mVs−1) without a diffusion limit. The surface redox contribution owing to the chemisorption of Na+ on the surface of MnO2 nanosheets is 30.8, 18.0, 20.0, 17.0, 13.5Fg−1 at 10, 25, 50, 75, and 100mVs−1, respectively. The intercalation contribution thanks to the solvated Na+ insertion into the crystalline bulk structure of layered MnO2 is 96.4, 57.1, 38.6, 30.5, and 25.3Fg−1 at 10, 25, 50, 75, 100mVs−1, respectively. The surface redox reaction and intercalation are still under diffusion limit. Understanding the charge storage mechanism/contribution of MnO2 nanosheets may lead to further development of these materials for practical applications.