A facile activation strategy can transform pristine carbon fiber tows into high-performance fiber electrodes with a specific capacitance of 14.2 F cm(-3). The knottable fiber supercapacitor shows an energy density of 0.35 mW h cm(-3), an ultrahigh power density of 3000 mW cm(-3), and a remarkable capacitance retention of 68%, when the scan rate increases from 10 to 1000 mV s(-1).