In this research magnesium dioxide (MnO₂) is electrodeposited over activated carbon paper (ACP) to form a composited MnO₂/ACP material. The as-prepared MnO₂/ACP shows excellent capacitance performance with a high specific capacitance of 485.4 F g⁻¹ calculated from a discharge curve with current density 2.0 A g⁻¹, owing to its enlarged specific surface area and improved electronic conductivity. Moreover, the MnO₂/ACP possesses remarkable rate capability due to the easy access of electrolytic ions, leading to complete utilization of MnO₂ active material for supercapacitors. To summarize, the electrodeposition of MnO₂ thin film on activated carbon paper is reported for the first time, and the composited MnO₂/ACP is a promising electrode material for building up efficient supercapacitors.