High-surface-area porous carbon materials with high porosity and well-defined pore structures are the preferred advanced supercapacitors electrode materials. Here, we report the electrochemical supercapacitive performance of novel high-porosity activated carbon materials prepared from biowaste Terminalia chebula (Harro) seed stones involving zinc chloride (ZnCl2) activation. Activation is achieved by mixing ZnCl2 with Harro seed powder (1:1 w/w) followed by carbonization at 400–700 °C under a nitrogen gas atmosphere. The amorphous carbon materials obtained exhibit excellent performance as electrical double-layer capacitor electrodes in aqueous electrolyte (1 M sulfuric acid) due to high specific surface areas (as high as 1382.6 m2 g−1) based on well-developed micropore and mesopore structures, and partial graphitic structure containing oxygenated surface functional groups. An electrode prepared using material having the optimal surface textural properties achieved a large specific capacitance of 328.6 F g−1 at 1 A g−1 in a three-electrode cell setup. The electrode achieved a good capacitance retention of 44.7% at a high 50 A g−1 current density and outstanding cycling performance of 98.2% even following 10,000 successive charge/discharge cycles. Electrochemical data indicate the significant potential of Terminalia chebula seed-derived porous carbons as high-performance electrode materials for high-energy-storage supercapacitor applications.