Activated carbon (AC) derived from biomass lightweight cork (Quercus suber) material was synthesized by KOH activation with different mass ratios of Q. suber: KOH in order to investigate the electrochemical properties of the AC in relation to KOH concentration. A well-defined porous activated carbon was obtained with a high surface area of 1081 m2 g−1 and a high pore volume of 0.66 cm3 g−1 when the Q. suber: KOH mass ratio was fixed at 1:2. A specific capacitance of 166 F g−1 was obtained for the symmetric device at 0.5 A g−1 in 1 M Na2SO4 with energy and power densities of 18.6 and 449.4 W Kg−1, respectively. The device displays good cycling stability after floating test for 200 h at 1.8 V and also displays 99.8% capacitance retention after cycling for 5000 cycles. The excellent electrochemical performance of the device makes it a potential material for supercapacitor application.