Carbon materials have been widely used as electrode materials for supercapacitors, while the current carbon precursors are mainly derived from fossil fuels. Biomass-derived carbon materials have become new and effective materials for electrodes of supercapacitors due to their sustainability, low pollution potential, and abundant reserves. Herein, we present a new biomass carbon material derived from water hyacinth by a novel activation method (combination of KOH and HNO activation). According to the electrochemical measurements, the material presents an ultrahigh capacitance of 374 F g (the current density is 1 A g). Furthermore, the material demonstrates excellent rate performance (105 F g at a higher density of 20 A g) and ideal cycling stability (87.3% capacity retention after 5000 times charge–discharge at 2 A g). When used for a symmetrical supercapacitor device, the material also shows a relatively high capacity of 330 F g at 1 A g (a two-electrode system). All measurements suggest the material is an effective and noteworthy material for the electrodes of supercapacitors.