A hydration-layered structure of buserite-type manganese oxide (Mg–buserite) was successfully synthesized by an ion exchange method. The as-prepared Mg–buserite possesses a large basal spacing of 10 Å, and contains Mg2+ ions and two sheets of water molecules in the interlayer region. The supercapacitive behaviors of Mg–buserite were systematically investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (CD) experiments and electrochemical impedance spectroscopy (EIS). The results showed that the specific capacitance of the Mg–buserite electrode sharply increased during the initial 500 cycles and reached a maximum of 164 F g−1 at approximately the 500th cycle at a scan rate of 1 mV s−1, and then it remained an almost constant value and decreased slightly upon prolonged cycling. After 22,000 cycles, the specific capacitance decreased by approximately 6% of the maximum specific capacitance. The superior capacitive behavior and excellent cycling stability of the as-prepared Mg–buserite are attributed to the large basal spacing, which can accommodate a larger amount of electrolyte cations and provide more favorable pathways for electrolyte cations intercalation and deintercalation. The experimental results demonstrate that Mg–buserite is a promising candidate as an electrode material for supercapacitors.