Abstract: In this study, we employed a one-step hydrothermal method to in situ grow spherical NiS2 nanoparticles on the surface of MXene, successfully constructing a NiS2-MXene hybrid composite. This study demonstrates that the integration of a NiS2-MXene hybrid composite into MgH2 substantially improves its hydrogen storage performance. Specifically, the composite reduces the initial dehydrogenation temperature of MgH2 by 118 °C, lowering it from 310 °C (pure MgH2) to 192 °C. At 300 °C, it can release 5.87 wt.% of hydrogen within 12 minutes. Furthermore, it demonstrates the ability to absorb hydrogen under ambient temperature conditions, with approximately 2.96 wt.% of hydrogen being absorbed as the temperature increases from room temperature to 50 °C. The activation energies for hydrogenation and dehydrogenation of the NiS2-MXene-MgH2 composite reduced by 33.6 kJ∙mol-1 and 40.7 kJ∙mol-1, respectively, in comparison to those of pure MgH2. Mechanistic studies demonstrate that NiS2-MXene enhances hydrogen storage performance through multiple synergistic effects. Specifically, the multivalent titanium in MXene establishes efficient electron transport pathways, promoting hydrogen binding and dissociation. Moreover, the in situ formation of Mg2Ni/Mg2NiH4 and MgS creates numerous phase interfaces, offering abundant active sites that facilitate both the dissociation and recombination of hydrogen molecules. Furthermore, the high specific surface area of MXene effectively inhibits agglomeration between the catalyst and Mg/MgH2, thereby maintaining structural stability and reactivity.