Lignocellulosic biomass such as plants and agricultural waste are ideal to tackle the current energy crisis and energy-related environmental issues. Carbon-rich lignin is abundant in lignocellulosic biomass, whose high-value transformation and utilization has been the most urgent problem to be solved. Herein, we propose a method for the preparation of porous carbon from lignin employing an H₃PO₄-assisted hydrothermal method. We characterize the as-prepared lignin-derived porous carbon and investigate its potential for energy storage. After assisted hydrothermal treatment followed by carbonization at 800 °C, the lignin-derived porous carbon displays a high specific capacitance (223.6 F·g^–1 at 0.1 A·g^–1) and excellent cycling ability with good capacitance retention. In this present study, the resultant lignin-derived porous carbon was used as the electrode of a supercapacitor, illustrating yet another potential high-value use for lignin, namely as a candidate for the sustainable fabrication of main supercapacitor components.