The development of an effective route to high-performance carbonaceous electrode materials derived from low-cost biomass is critical but remains challenging for supercapacitors. Here we propose a new and cost-effective way to produce chitosan-based hierarchical porous carbons by the union of hydrothermal carbonization and chemical activation. The key to this preparation strategy is the utilization of acetic acid as an additive for hydrothermal carbonization, which not only favors the construction of a conducive environment for accessibility of activator KOH, but also leads to the formation of a rigid semi-carbonized framework substrate for generating an ultrahighly porous structure. Thus, our synthetic approach allows for a lower amount of activation agent and lower heating temperature when compared with normal chemical activation techniques, providing a more efficient way to produce ultrahigh-surface-area carbon materials. The as-prepared hierarchical porous carbon possesses a unique honeycomb-like framework and the highest BET surface area of 3532 m² g⁻¹ among all the carbon materials derived from chitosan. The combination of the hierarchical pore structure for rapid ion diffusion and the ultrahigh surface area for sufficient electrochemically active sites significantly improves the material's capacitive behaviors. An unusually high capacitance of 455 F g⁻¹, an excellent cycling stability with 99% capacity retention over 20 000 cycles in KOH aqueous electrolyte, and a high energy density of 20.6 W h kg⁻¹ at a power density of 226.8 W kg⁻¹ in 1.8 V Na₂SO₄ aqueous supercapacitors have been obtained, demonstrating that the chitosan-based hierarchical porous carbons developed here are very attractive for application in supercapacitors.