Abstract Here we report the methylene blue adsorption and energy storage supercapacitance performances of the nanoporous activated carbons obtained by the zinc chloride (ZnCl2) activation of biowaste, Terminalia bellirica (Barro) seed stone. The activation was performed at lower temperatures (400–700 °C) under an inert nitrogen gas atmosphere. The total specific surface area and pore volume range from 1077 to 1303 m2 g−1 and 0.752 to 0.873 cm3 g−1, depending on the carbonization temperature. Due to the well-developed porosity, the sample with optimal surface area showed excellent iodine and methylene blue adsorption properties with a maximum iodine number and methylene blue value of 909.8 mg g−1 and 357.2 mg g−1, respectively. Batch adsorption studies revealed that the optimum methylene blue adsorption is favorable in an alkaline medium, with a contact time of 270 min and an adsorbent dose of 8 g L−1, respectively. The Langmuir isotherm model could best explain the equilibrium adsorption with a monolayer adsorption capacity of 312.5 mg g−1. The electrochemical measurements performed in a three-electrode system revealed a high specific capacitance of 319 F g−1 at 1 A g−1. Furthermore, the electrode retained 46% capacitance at 50 A g−1 with an excellent cycle life of 98.5% after 10,000 consecutive charging/discharging cycles. These results imply that a biowaste Terminalia bellirica seed has a considerable potential to produce high surface area porous carbons materials desired in adsorption technology and high-performance supercapacitor applications.