Sodium‐ion hybrid supercapacitors are potential energy‐storage devices and have recently received enormous interest. However, the development of cathode materials and the use of nonaqueous electrolyte remain a great challenge. Hence, aqueous Na‐ion hybrid supercapacitors based on a three‐dimensional network of NaMnO2 were developed. The cathode material was synthesized by the electro‐oxidation of potassium manganese hexacyanoferrate nanocubes. The oxidized compound was confirmed to be Na0.21MnO2 by various physical characterization methods. Manganese dioxide is a well‐characterized material for aqueous asymmetric pseudocapacitors, but its usage at high operating voltages is limited due to the electrochemical stability of water. Nevertheless, high‐potential and high‐performance aqueous supercapacitors exhibiting a cell potential of 2.7 V were developed. Further, the practical applicability of an asymmetric supercapacitor based on NaMnO2 (cathode) and reduced graphene oxide (anode) was demonstrated by powering a 2.1 V red LED.