Manganese oxide is considered an ideal pseudo-capacitive electrode material for supercapacitors due to its low cost, environmental friendliness and large theoretical capacity. However, it is difficult to obtain manganese electrodes with a high specific capacitance and a large voltage range. In this study, ultrafine Mn3O4 nanowires with an average diameter of 4.0 nm were synthesized using a colloidal method. They have a large specific surface area of 175.1 m2 g−1, and can provide numerous active sites to enhance their specific capacitances. They also show a large pore volume of 0.7960 cm3 g−1, which can provide essential channels for ion transport during charging and discharging processes. The supercapacitor electrode made of these ultrafine Mn3O4 nanowires exhibits a predominant surface capacitive behavior during charge/discharge processes, and achieves a large specific capacitance of 433.1 F g−1 at a current density of 0.5 A g−1 with a very wide voltage range from -0.5 to 1.1 V in 1 M Na2SO4 electrolyte. An asymmetric supercapacitor (ASC) was assembled using a cathode electrode made of these ultrafine Mn3O4 nanowires and an active carbon (AC) anode electrode, and a high energy density of 26.68 Wh kg−1 at a power density of 442 W kg−1 was achieved. The ASC showed a good cycling stability, and its capacitance value was still maintained at 75.8% after 64,000 charge/discharge cycles.