Dry-spun Carbon Nanotube (CNT) fibers were surface-modified by atmospheric pressure oxygen plasma functionalization using a well controlled and continuous process. The fibers were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). It was found from the conducted electrochemical measurements that the functionalized fibers showed a 132.8% increase in specific capacitance compared to non-functionalized fibers. Dye-adsorption test and the obtained Randles-Sevcik plot demonstrated that the oxygen plasma functionalized fibers exhibited increased surface area. It was further established by Brunauer-Emmett-Teller (BET) measurements that the surface area of the CNT fibers was increased from 168.22 m2/g to 208.01 m2/g after plasma functionalization. The pore size distribution of the fibers was also altered by this processing. The improved electrochemical data was attributed to enhanced wettability, increased surface area, and the presence of oxygen functional groups, which promoted the capacitance of the fibers. Fiber supercapacitors were fabricated from the oxygen plasma functionalized CNT fiber electrodes using different electrolyte systems. The devices with functionalized electrodes exhibited excellent cyclic stability (93.2% after 4000 cycles), flexibility, bendability, and good energy densities. At 0.5 mA/cm2, the EMIMBF4 device revealed a specific capacitance, which is 27% and 65% greater than the specific capacitances of devices using EMIMTFSI and H2SO4 electrolytes, respectively. The practiced in this work plasma surface processing can be employed in other applications where fibers, yarns, ribbons, and sheets need to be chemically modified.