The integration of 2D graphene nanosheets and layered transition metal dichalcogenides has been recognized as one of the most extensive strategies for obtaining promising electrode materials for energy storage devices. Cubic manganese diselenide (MnSe2) and rGO-decorated MnSe2 (G-MnSe2) hybrid materials were synthesized via a facile hydrothermal method. The development of renewable energy conversion systems requires efficient electrochemical energy storage devices. Among the various electrochemical energy storage devices, the supercapacitor is considered one of the most promising energy storage devices owing to its excellent electrochemical properties, such as high power density, rapid charging rate, and long-term cycling stability. The key performance of the supercapacitor is based on the energy storage capacity of the electrode materials and the stability of the electrolyte for a wide range of operating voltages. Carbon-based materials are the best-known double-layer capacitance electrodes, but the fundamental shortcoming is their low specific capacitance and inferior energy density.