The pursuit of new categories of active materials as electrodes of supercapacitors remains a great challenge. Herein, for the first time, elemental boron as a superior electrode material of supercapacitors is reported, which exhibits significantly high capacitances and excellent rate performance in all alkaline, neutral, and acidic electrolytes. Notably, boron nanowire-carbon fiber cloth (BNWs-CFC) electrodes achieve a capacitance up to 42.8 mF cm-2 at a scan rate of 5 mV s-1 and 60.2 mF cm-2 at a current density of 0.2 mA cm-2 in the acidic electrolyte. Moreover, in all these three kinds of electrolytes, BNWs-CFC electrodes demonstrate a decent cycling stability with >80% capacitance retention after 8000 charging/discharging cycles. The Dominating energy storage mechanism of BNWs in the different electrolytes is analyzed by looking into the kinetics of the electrochemical process. Subsequently, the BNWs-CFC electrode is used to fabricate a flexible solid-state supercapacitor, which reveals a specific capacitance up to 22.73 mF cm-2 and good mechanical performance after 1000 bending cycles. This study opens a new avenue to explore elemental boron-based new nanomaterials for the application of energy storage with superior electrochemical performance.