NiOOH nanosheet/graphene hydrogels (H–NiOOH/GS), with mesoporous NiOOH nanosheets uniformly dispersed within the highly interconnected 3D graphene network, are constructed and studied for the first time by a mixed solvothermal and hydrothermal reaction. The effect of solvent composition on the morphology, phase, dispersibility of nanocrystal and hydrogel strength is systematically studied. As binder-free electrodes of supercapacitors, H–NiOOH/GS delivers high capacitance of 1162Fg−1 at 1Ag−1 with excellent rate capability (981Fg−1 at 20Ag−1). The charge-storage mechanisms of H–NiOOH/GS are in-depth investigated by quantifying the kinetics of charge storage, which reveals that NiOOH exhibits both capacitive effects and diffusion-controlled battery-type behavior during charge storage. Additionally, solvothermal-induced pure graphene hydrogels (H-GS) are also prepared and used as the negative electrode for the first time, which show an impressive specific capacitance of 425 and 368Fg−1 at 5 and 40mVs−1, respectively. Benefitting from the synergistic contribution of both positive and negative electrodes, the assembled H–NiOOH/GS//H-GS asymmetric supercapacitors achieve a remarkable energy density of 66.8Whkg−1 at a power density of 800Wkg−1, and excellent cycling stability with 85.3% capacitance retention after 8000 cycles, holding great promise for energy storage applications.