We demonstrate the synthesis of a composite of α-MnO2 nanorods and dielectric boron nitride nanoplatelets(BNNPs) as an electrode material for application in nanoscale dielectric pseudocapacitors. The optimize nanocomposite delivers a significantly high specific capacitance (890 F/g at 0.41 A/g), which is >72% of the theoretical specific capacitance of α-MnO2 at a mass loading of ca. 6 wt%. Pure BNNPs exhibit negligible charge storage capacity with a specific capacitance of 1.30 F/g at 0.024 A/g. The BNNPs increase the amount of K+ insertion/extraction and the conductivity of α-MnO2 nanorods by lowering the charge transfer resistance at the electrode-electrolyte interface. This is due to the electrical polarization of dielectric BNNPs during charging and discharging, which increases the rate and amount of K+ insertion or extraction induce by electrostatic force. The nanocomposite shows good capacity retention (94.12% after 2000 cycles) with high energy and power density. This research opens up a new avenue for the development of new types of nanoscale dielectric pseudocapacitors with high capacitance by exploring other suitable metal-oxides and nanoscale dielectric material composites.