This study investigates the dynamic restructuring of nickel sulfides (NiS) under alkaline conditions for the electrocatalytic hydrogen evolution reaction (HER). Using operando X-ray absorption spectroscopy (XAS) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), the research reveals that NiS undergoes an in-situ phase transition to a mixed phase of Ni₃S₂ and NiO, forming a highly active synergistic dual site interface. The interfacial Ni sites facilitate water dissociation and OH* adsorption, while the interfacial S sites promote H* adsorption and H₂ evolution. This in-situ formation of the Ni₃S₂/NiO interface enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm⁻². The study highlights the dynamic chemistry of transition metal chalcogenides and demonstrates that careful control of working conditions can lead to the in-situ formation of catalytic species that enhance catalytic performance. The results show that the Ni₃S₂/NiO interface provides dual active sites for water dissociation and H₂ evolution, with the interfacial Ni sites accelerating the rate-determining step of water dissociation and the interfacial S sites facilitating H* adsorption. Theoretical calculations further support the mechanism, showing that the Ni₃S₂/NiO interface enhances HER activity by reducing the energy barrier for water dissociation and promoting H* coupling to form H₂. The study also demonstrates the long-term stability of the Ni₃S₂/NiO interface under HER conditions. Overall, the research provides insights into the dynamic restructuring of NiS under alkaline HER conditions and highlights the importance of interfacial engineering for enhancing catalytic performance.This study investigates the dynamic restructuring of nickel sulfides (NiS) under alkaline conditions for the electrocatalytic hydrogen evolution reaction (HER). Using operando X-ray absorption spectroscopy (XAS) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), the research reveals that NiS undergoes an in-situ phase transition to a mixed phase of Ni₃S₂ and NiO, forming a highly active synergistic dual site interface. The interfacial Ni sites facilitate water dissociation and OH* adsorption, while the interfacial S sites promote H* adsorption and H₂ evolution. This in-situ formation of the Ni₃S₂/NiO interface enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm⁻². The study highlights the dynamic chemistry of transition metal chalcogenides and demonstrates that careful control of working conditions can lead to the in-situ formation of catalytic species that enhance catalytic performance. The results show that the Ni₃S₂/NiO interface provides dual active sites for water dissociation and H₂ evolution, with the interfacial Ni sites accelerating the rate-determining step of water dissociation and the interfacial S sites facilitating H* adsorption. Theoretical calculations further support the mechanism, showing that the Ni₃S₂/NiO interface enhances HER activity by reducing the energy barrier for water dissociation and promoting H* coupling to form H₂. The study also demonstrates the long-term stability of the Ni₃S₂/NiO interface under HER conditions. Overall, the research provides insights into the dynamic restructuring of NiS under alkaline HER conditions and highlights the importance of interfacial engineering for enhancing catalytic performance.