Transition metal selenides (TMSs) with excellent electrochemical activity and high intrinsic electrical conductivity have attracted considerable attention owing to their potential use in energy storage devices. However, the low energy densities of the reported TMSs, which originate from the small active surface area and poor electrolyte ion mobility, substantially restrict their application potential. In this work, porous ultrathin nickel selenide nanosheet networks (NiSe NNs) on nickel foam are fabricated by using a novel, facile method, that is, selenylation/pickling of the pre-formed manganese-doped α-Ni(OH) ₂. Removal of Mn resulted in NNs with a highly porous structure. The 3D framework of the NNs and the inherent nature of the NiSe affords high ion mobility, abundant accessible activated sites, vigorous electrochemical activity, and low resistance. One of the highest specific capacities of TMSs ever reported, that is, 443 mA h g ⁻¹ (807 μAh cm ⁻²) at 3.0 A g ⁻¹, is achieved with the NNs as electrodes. The assembled NiSe NNs//porous carbon hybrid supercapacitor delivers a high energy density of 66.6 Wh kg ⁻¹ at a power density of 425 W kg ⁻¹, with excellent cycling stability. This work provides a new strategy for the production of novel electrode materials that can be applied in high-performance hybrid supercapacitors, and a fresh pathway towards commercial applications of hybrid supercapacitors based on TMS electrodes.