Green and renewable organic redox molecules are greatly advantageous over conventional inorganic intercalation electrode materials in terms of electrochemical reversibility and cycling stability. However, their electrical insulation prevents them from being used alone as electrode materials for supercapacitors. Herein, 2-amino-3-chloro-1,4-naphthoquinone (ACNQ) molecules were covalently grafted onto graphene nanosheets (GNS) via diazotization. The ACNQ-functionalized GNS (CNQ-GNS) electrode material exhibited a high specific capacitance of 364.2 ± 10 F g-1 at a current density of 1 A g-1, which is much larger than that of bare GNS (190 ± 6 F g-1). Moreover, the electrode exhibited an outstanding rate capability (capacitance retention of 76.8% at 100 A g-1). Finally, an asymmetric supercapacitor device was fabricated using graphene nanosheets as the positive electrode and the optimized CNQ-GNS as the negative electrode, which displayed a high energy density of 19.1 W h kg-1 at a power density of 0.8 kW kg-1 with a long cycling life span (nearly no loss after 10 000 cycles at 5 A g-1) in 1 M H2SO4 electrolyte. Briefly, the highly conductive GNS scaffold delivers a high electrical double-layer capacitance, while the organic functional groups covalently bonded on the GNS contribute additional faradaic pseudocapacitance, resulting in outstanding electrochemical energy storage.