Nanocomposites of interpenetrating carbon nanotubes and vanadium pentoxide (V2O5) nanowires networks are synthesized via a simple in situ hydrothermal process. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which makes them excellent material candidates for supercapacitors with high energy density and power density. Nanocomposites with a capacitance up to 440 and 200 F g−1 are achieved at current densities of 0.25 and 10 A g−1, respectively. Asymmetric devices based on these nanocomposites and aqueous electrolyte exhibit an excellent charge/discharge capability, and high energy densities of 16 W h kg−1 at a power density of 75 W kg−1 and 5.5 W h kg−1 at a high power density of 3 750 W kg−1. This performance is a significant improvement over current electrochemical capacitors and is highly competetive with Ni–MH batteries. This work provides a new platform for high-density electrical-energy storage for electric vehicles and other applications.