Carboxylated graphene–ZnO (G-COOZn) composites were grown as ZnO nanoparticles onto graphene sheets by a one-step thermal method using carboxylated graphene (G-COOH) and Zn(NO3)2. G-COOH sheets were synthesized from graphene oxide (GO) and chloroacetic acid (Cl–CH2–COOH). GO was used as the starting material, as prepared by the Hummers’ method using graphite flakes. GO, G-COOH, and G-COOZn were characterized by XRD, AFM, SEM, TEM, FT-IR, 13C-NMR, and Raman spectroscopy. The electrochemical properties of the G-COOZn supercapacitor were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within the potential range of 0–1V. The supercapacitor delivered a specific capacitance of ∼238Fg−1 at a current density of 50mAcm−2. This method provides an easy and straightforward approach to deposit ZnO nanoparticles onto graphene sheets, and may be readily extended to the preparation of other classes of hybrids based on GO sheets for specific technological applications.