We report a practical and efficient strategy for synthesizing hierarchical (meso- and macro-)porous NiO nano/micro spherical superstructures. First, β-Ni(OH)₂ microspheres were self-assembled based on the coalescence and Ostwald-ripening mechanisms during refluxing in an alkaline solution with Ni(NH₃)_x²⁺ at 97 °C for 1 h under vigorous stirring. Second, hierarchical porous NiO microsphere superstructures were obtained from the precursor by a simple calcination procedure. The resulting superstructures comprised two-dimensional mesoporous NiO petal building blocks. Electrochemical data demonstrated that the hierarchical porous NiO nano/micro superstructures were capable of delivering a specific capacitance of 710 F g⁻¹ at 1 A g⁻¹ and offered a good specific capacitance retention of ca. 98% after 2000 continuous charge-discharge cycles. This indicates that we successfully met key requirements in terms of large specific energy density, high-rate capability and good electrochemical stability. We would expect our superstructures to be good candidates for a low-cost replacement for the state-of-the-art supercapacitor material RuO₂.