Electrodes composed of ultrathin MnO2 (thickness 5-80 nm) spines on Au nanowire (NW) stems (length 10-20 μm, diameter 20-100 nm) were electrochemically grown on flexible polyethylene terephthalate (PET) substrates. The electrodes demonstrated high specific capacitance, high specific energy value, high specific power value, and long-term stability. In Na 2SO4(aq.) (1 M), the maximum specific capacitance was determined to be 1130 F g-1 by cyclic voltammetry (CV, scan rate 2 mV s-1) using a three-electrode system. From a galvanostatic (GV) charge/discharge test using a two-electrode system, a maximum capacitance of 225 F g-1 (current density 1 A g-1) was measured. Even at a high charge/discharge rate of 50 A g-1, the specific capacitance remained at an extremely high value of 165 F g-1. The flexible electrodes also exhibited a maximum specific energy of 15 W h kg-1 and a specific power of 20 kW kg-1 at 50 A g-1. After five thousand cycles at 10 A g-1, 90% of the original capacitance was retained. A highly flexible solid-state device was also fabricated to reveal its supercapacitance performance.