A core–shell nanostructured Mg@Pt composite, consisting many of icosahedral Mg particles as the core with nano-sized Pt particles distributed homogeneously on different surfaces, was synthesized through an arc plasma method followed by the generic solid-state method. The microstructures and hydrogen sorption properties of the Mg@Pt composite were carefully investigated and compared to that of pure Mg powder. In particular, the dehydrogenation behaviors of the hydrogenated Mg@Pt composite were observed in situ via high-resolution transmission electron microscopy (HRTEM). The results revealed that the Pt on Mg particles showed a “spillover” effect to improve the hydrogen absorption kinetics in the early stage. It then transformed into H-stabilized Mg₃Pt, followed by the formation of MgH₂. DFT calculation and in situ TEM observations demonstrated that the H-stabilized Mg₃Pt acted as a “hydrogen pump” for the dehydrogenation of MgH₂ and then transformed into Pt after desorption. Through this effect, the dehydriding kinetics of the hydrogenated Mg@Pt composite was improved and the onset dehydrogenation temperature was reduced compared to that of the pure MgH₂ powder.