In this work, a three-dimensional (3D) graphene/polyaniline (GNS/PANi) composite hydrogel was prepared via a self-assembled method followed by in situ polymerization of aniline. The chemical structure of the materials was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman. The PANi nanofibers homogeneously distribute on the surfaces of the GNSs or among the GNSs, forming a light-weight and 3D composite hydrogel. Due to the reinforcing effects coming from PANi, the capacitive performance of pure GNS hydrogel is significantly improved after adding PANi. The GNS/PANi composite hydrogel exhibits the largest specific capacitance of 334 F g⁻¹, which is about 150% higher than that of pure GNS hydrogel. In addition, the electrochemical properties of the samples are further studied at the high and low operating temperatures (60 and −5°C). Due to the thermal-instability of PANi, the specific capacitance and the cycle-stability of the GNS/PANi composite hydrogel show strong dependent on the operating temperature, especially at high temperature. Therefore, based on the above investigations, such GNS/PANi composite hydrogel would be a potential candidate for high-performance capacitor electrodes using at warm environment.