Numerous electrode materials have been studied in supercapacitors for next-generation energy storage applications. As a paramagnetic metal with low resistivity, NiSe2 has received much attention and been used extensively in many applications, including energy storage, electrocatalysts, and high temperature superconductors, etc. However, the capacitive properties of NiSe2 are rarely investigated. In the present work, truncated cube-like NiSe2 single crystals are synthesized by a facile hydrothermal approach and further used as electrode material in supercapacitors. The effects of different loading mass of electrode material on electro-chemical capacitive behaviors are also investigated. Experimental results demonstrate that under a mass loading of 3.90 mg cm(-2), the as-prepared NiSe2 electrode exhibits a high specific capacitance of 1044 F g(-1) at 3 A g(-1) (or an areal capacitance of 4.07 F cm(-2)), along with an excellent rate capability (601 F g(-1) at 30 A g(-1)). Besides, the morphology change and the impedance increasement are responsible for the worse cycling performance of NiSe2 electrode in the three-electrode system. Meanwhile, the practical electrochemical energy storage behavior of as-synthesized NiSe2 is investigated in an asymmetric supercapacitor. The NiSe2//activated carbon (AC) asymmetric device possesses an outstanding cycle life (87.4% after 20,000 successive cycles), a high energy density of 44.8 Wh kg(-1) at 969.7 W kg(-1) and a higher power density of 17.2 kW kg(-1) at 17.4 Wh kg(-1), showing attractive potential in practical applications. This work opens avenue for utilizing single crystal NiSe2 as electrode material and providing important guidance to the further investigation of nickel selenides for advanced supercapacitors.