Potassium-ion batteries (PIBs) are promising candidates for next-generation energy storage devices due to the earth abundance of potassium, low cost, and stable redox potentials. However, the lack of promising high-performance electrode materials for the intercalation/deintercalation of large potassium ions is a major challenge up to date. Herein, we report a novel uniform nickel selenide nanoparticles encapsulated in nitrogen-doped carbon (defined as pNiSe@NCq) as an anode for PIBs, which exhibits superior rate performance and cyclic stability. Benefiting from the unique hierarchical core-shell like nanostructure, the intrinsic properties of metal-selenium bonds, synergetic effect of different components, and a remarkable pseudocapacitance effect, the anode exhibits a very high reversible capacity of 438 mAdhdg^-1 at 50 mAdg^-1, an excellent rate capability, and remarkable cycling performance over 2, 000 cycles. The electrochemical mechanism were investigated by the in-situ X-ray diffraction, ex-situ high-resolution transmission electron microscopy, selected area electron diffraction, and first principle calculations. In addition, NiSe@NC anode also shows high reversible capacity of 512 mAdhdg^-1 at 100 mAdg^-1 with 84% initial Coulombic efficiency, remarkable rate performance, and excellent cycling life for sodium ion batteries. We believe the proposed simple approach will pave a new way to synthesize suitable anode materials for secondary ion batteries.