Orthorhombic molybdenum trioxide (alpha-MoO3) as an electrode material for electrochemical energy storage has attracted much attention due to its unique intercalation pseudocapacitive property. However, subjected to its low conductivity and intrinsic redox reaction, supercapacitors with alpha-MoO3 electrodes usually suffer from specific capacitance far below the theoretical value and poor cycling life. In this work, NiMoO4-modified alpha-MoO3 nanobelts are synthesized through a facile two-step hydrothermal method. The MoO3 nanobelts are fully covered by an ultrathin mesoporous NiMoO4 layer (10-20 nm), i.e. MoO3/NiMoO4 core/shell nanobelts. Benefiting from the high electrochemical activity of NiMoO4, the surface recombination effect between NiMoO4 and MoO3, the porous structure and abundant active sites in the NiMoO4 shell acting as "ion-buffering reservoirs", the MoO3/NiMoO4 electrodes exhibit a high specific capacitance of 1307 F g(-1) (nearly six times that of the pristine alpha-MoO3 electrode), good rate capability and excellent cycling performance in 3M KOH electrolyte. In addition, asymmetric supercapacitors are assembled with MoO3/NiMoO4 as positive electrodes and plant spore-derived activated carbon microspheres as negative electrodes. A maximum specific capacitance of 184 F g(-1) and an energy density of 37.5 Wh Kg(-1) are demonstrated within a cell voltage between 0 and 1.7 V; the device shows a remarkable cycling capability with 100% retention after 75,000 cycles.