Biomass-derived activated carbon materials were prepared by a two-step synthesis via carbonization followed by KOH activation of rice straw at 600 degrees C in an argon atmosphere. The formation of disordered micro- and mesopores on carbon by KOH chemical activation and the high specific surface area of similar to 1007 m(2) g(-1) were confirmed by N-2 adsorption-desorption. Further, the scanning electron microscopic analysis revealed the formation of disordered pores over the carbon surface, and the transmission electron microscopic analysis confirmed the formation and aggregation of ultrafine carbon nanoparticles of in size after the carbonization and activation processes. The three-electrode cell in aqueous electrolyte shows high specific capacitance of 332 F g(-1), with high specific capacitance retention of 99% after 5000 cycles. The fabricated symmetric supercapacitor device in aqueous 1 M H2SO4 electrolyte showed a high specific capacitance of 156 F g(-1), with a high energy density of 7.8 Wh kg(-1). The symmetric device fabricated using 1-ethyl-3-methyl imidazolium tetrafluoroborate ([EMIM][BF4]) ionic liquid exhibited a cell voltage of 2.5 V and a specific capacitance of 80 F g(-1), with a high energy density of 17.4 Wh kg(-1). The observed electrochemical performance clearly indicates that activated carbon derived from rice straw could be used as a promising electrode material in a supercapacitor for electrochemical energy storage. The cheaper and readily available rice straw raw materials, simple chemical activation process, and high performance promise that the obtained carbon material is viable for commercial applications in supercapacitors.