20 nm sized Co3O4 nanoparticles are in-situ grown on the chemically reduced graphene oxide (rGO) sheets to form a rGO–Co3O4 composite during hydrothermal processing. The rGO–Co3O4 composite is employed as the pseudocapacitor electrode in the 2 M KOH aqueous electrolyte solution. The rGO–Co3O4 composite electrode exhibits a specific capacitance of 472 F g−1 at a scan rate of 2 mV s−1 in a two-electrode cell. 82.6% of capacitance is retained when the scan rate increases to 100 mV s−1. The rGO–Co3O4 composite electrode shows high rate capability and excellent long-term stability. It also exhibits high energy density at relatively high power density. The energy density reaches 39.0 Wh kg−1 at a power density of 8.3 kW kg−1. The super performance of the composite electrode is attributed to the synergistic effects of small size and good redox activity of the Co3O4 particles combined with high electronic conductivity of the rGO sheets.