To achieve superior performance of electrode materials, the design of rational and advantageous hierarchical structures has been confirmed as an effective and feasible approach. Herein, we report an effective strategy for the synthesis of hierarchical NiSe₂ spheres composed of nanoparticles: Ni(OH)₂ flowers were firstly prepared as the precursor via a solvothermal method followed by a solvothermal selenylation reaction to prepare hierarchical NiSe₂ spheres. Investigated as electrode materials for asymmetric supercapacitors, the NiSe₂ spheres//AC device exhibits enhanced electrochemical performance with a high specific capacitance of 99.7 F g⁻¹ at a current density of 2 A g⁻¹. In addition, high energy/power density and long-term cycling stability over 10 000 cycles have been shown, which derive from the nanosized building-blocks and secondary micro-architecture.