Nickel hydroxide has shown extraordinary promise for hybrid supercapacitors because of the high theoretical capacity and low cost, but is usually limited by its low power density and complex synthetic process. Here, we report the design of a self-assembly three-dimensional (3D) porous Ni(OH)2 for high-performance energy storage through a simple, cost-effective, and environmentally friendly solvothermal-precipitation method. The 3D structure with interconnected network provides superior electron transportation properties, and the hierarchical porous architecture facilitates quick ion diffusion. The Ni(OH)2 obtained at 250 °C delivers a high capacitance of 2110 F g−1 at 1.0 A g−1 and a moderate cycling stability with 53% of the maximum specific capacitance over 2000 cycles at 5.0 A g−1. Additionally, on integration of the prepared Ni(OH)2 with hierarchically porous carbons as negative electrode material, the hybrid supercapacitor device delivers a high capacitance of 115 F g−1 and a high energy density of 40.9 W h kg−1 at a power density of 405 W kg−1. This study not only provides a simply controllable growth method of the unique self-assembly 3D porous Ni(OH)2 with high performance but also opens up a new approach to prepare various metallic oxides/hydroxides with excellent performance for highly promising practical energy-storage systems.