P-doped NiSe2 nanoparticles are encapsulated into CNTs decorated carbon frameworks. The introduction of P reduced the energy barriers for both water dissociation and the intermediates adsorption during alkaline HER and OER, thereby accelerating the water splitting kinetically. Display omitted •P-NiSe2 nanoparticles were encapsulated into hierarchical N-doped carbon frameworks.•DFT calculations reveal the P doping reduced the energy barriers for water dissociation.•P-NiSe2@N-CNFs/NC exhibits superior overall water splitting performance and stability. Nickel diselenide (NiSe2) is a promising low-cost catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), due to its suitable d-electron configuration and high electrical conductivity. Several representative elements, e.g., Co, Fe and P, have been utilized as cation or anion to promote the electrocatalytic activity of NiSe2 by modulating the interaction with Se element, whilst the catalyst stability is less concerned. In this work, the catalytic Ni nanoparticles were in-situ encapsulated in carbon nanotubes grown on three-dimensional conducting carbon framework. Subsequent phosphorization and selenization yield dispersed P-doped NiSe2 nanoparticles protected by carbon shell with highly exposed yet stable active sides, resulting in significantly promoted HER and OER activities as well as accelerated kinetics. In detail, the P-NiSe2@N-CNTs/NC hybrid catalyst deliver low overpotentials of 95 and 306 mV at 10 mA cm−2 for HER and OER in alkaline media, respectively. DFT calculations reveal that P doping reduces the electron density surrounding Ni atoms while accumulates the charges to Se, respectively, which in turn reduces the energy barriers for both water dissociation and intermediates adsorption for both HER and OER. As a concept of proof, a cell assembled by P-NiSe2@N-CNTs/NC hybrid catalyst-based anode and cathode performs a low applied voltage of 1.609 V to reach 10 mA cm−2, and outstanding long-term stability.