Metal oxides are a class of promising electrode materials for supercapacitors because of their high theoretical energy density; however, the low electrical conductivity and instability of metal oxides limit their large-scale practical applications. Here we report a facile and scalable method to synthesize ultrafine nickel–cobalt alloy nanoparticles (5–10 nm) embedded into three-dimensional porous graphitic carbon (3D Ni–Co@PGC) using NaCl as the template to create a porous structure and glucose as the carbon source by pyrolysis treatment at 800 °C under N₂ atmosphere. As an electrode material for supercapacitors, the ultrafine Ni–Co alloy nanoparticles of 3D Ni–Co@PGC not only serve as current collectors, but also their surfaces convert to corresponding metal oxides when exposed to an alkaline electrolyte, responsible for redox reactions in pseudocapacitors, exhibiting high supercapacitor performance. The results demonstrate that the supercapacitor assembled with 3D Ni–Co@PGC electrodes shows high energy density (1091, 1064 and 1041 F g⁻¹ at 1, 2 and 4 A g⁻¹, respectively), long cycling life and excellent rate capability at a high charge/discharge current. Furthermore, an asymmetric supercapacitor assembled by using 3D Ni–Co@PGC as the positive electrode and active carbon as the negative electrode shows a high energy density of 33.7 W h kg⁻¹ and remarkable cycling stability (98% capacitance retention over 4000 cycles). The superior performance of the 3D Ni–Co@PGC constructed supercapacitor can be ascribed to its high surface area (265 m² g⁻¹), porous structure and excellent electrical conductivity, favourable for the exposure of reaction active sites, redox-related mass transport and electron transfer, respectively.