Surface-modified RuO2 nanoparticles–carbon nanofiber (CNF) composites are synthesized using electrospinning and acid treatment in sequence, and their structure, morphology, chemical states, and electrochemical performance are demonstrated. The surface-modified RuO2–CNF composites exhibited the highest specific capacitance of 224.6Fg−1, high-rate performance with capacitance retention of 80%, superior energy density of 26.9–21.5Whkg−1, and excellent cycling stability of 90% after up to 3000 cycles, compared to the conventional CNFs, surface-modified CNFs, and unmodified RuO2 nanoparticles–CNF composites. Their improved electrochemical performance can be explained as synergistic effect of well-distributed RuO2 nanoparticles within the matrix of CNFs and surface modification induced by the increased number of oxygen-containing functional groups, which results in increased capacitance and improved high-rate performance due to Faradaic redox reactions and improved wettability.