Mn3O4/Ni(OH)2 nanocomposites are simply synthesized by a hydrothermal reaction between MnCl2 .4H2O, NiCl2.6H2O and NaOH at 200°C. Compared with the individual Ni(OH)2 and Mn3O4 prepared under the same conditions, the Mn3O4/Ni(OH)2 composite hydrothermally treated for 5h achieves excellent specific capacitance (707Fg−1 at 1Ag−1 in 1M KOH electrolyte) and remarkable long-term cycling stability (retaining a capacitance retention of 89% after 2000 cycles at 2Ag−1). Meanwhile, the asymmetric supercapacitor constructed by the Mn3O4/Ni(OH)2 composite and active carbon delivers an energy density of 17.8Whkg−1 at a power density of 162Wkg−1. The outstanding performance is attributable to the synergistic compositing effect of Mn3O4 and Ni(OH)2, yielding not only the mutual doping of Mn3+ in Ni(OH)2 and Ni2+ in Mn3O4 to improve the electronic conductivity of the Mn3O4/Ni(OH)2 composite but also the suitable mesoporous structure for electrolyte transfer. Thus the Mn3O4/Ni(OH)2 composites will be alternative candidates as practical electrode materials for pseudocapacitors.