Development of active materials capable of delivering high specific capacitance is one of the present challenges in supercapacitor applications. Herein, we report a facile and green solvothermal approach to synthesize graphene supported tungsten oxide (WO3) nanowires as an active electrode material. As an active electrode material, the graphene–WO3 nanowire nanocomposite with an optimized weight ratio has shown excellent electrochemical performance with a specific capacitance of 465 F g–1 at 1 A g–1 and a good cycling stability of 97.7% specific capacitance retention after 2000 cycles in 0.1 M H2SO4 electrolyte. Furthermore, a solid-state asymmetric supercapacitor (ASC) was fabricated by pairing a graphene–WO3 nanowire nanocomposite as a negative electrode and activated carbon as a positive electrode. The device has delivered an energy density of 26.7 W h kg–1 at 6 kW kg–1 power density, and it could retain 25 W h kg–1 at 6 kW kg–1 power density after 4000 cycles. The high energy density and excellent capacity retention obtained using a graphene–WO3 nanowire nanocomposite demonstrate that it could be a promising material for the practical application in energy storage devices.