Reduced graphene nanosheet/urchin-like manganese dioxide (GNS/MnO2) composite for a supercapacitor electrode has been fabricated by a mild hydrothermal route. Following investigation by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we propose an in situ formation of MnO2 nanoparticles onto graphene nanosheets. The unique structure greatly increases specific surface area of the composite and the utilization of MnO2. The electrochemical performance of the electrode is analyzed by cyclic voltammetry, electrochemical impedance spectrometry and chronopotentiometry. Results show that the GNS/MnO2 composite exhibits a maximum specific capacitance of 263Fg−1 and an excellent cycle life with capacity retention of about 99% after 500 cycles. The method provides a facile and straightforward approach to deposit MnO2 nanoparticles onto graphene sheets; it could be readily extended to the preparation of other classes of hybrids based on GNS sheets for technological application.