Abstract: The dissociative adsorption of H₂ on Cu₁₉ and defective graphene-supported Cu₁₉ clusters (Cu₁₉G) are investigated using ab initio molecular dynamics. The molecular-level trajectories show that, on Cu₁₉, the preferred adsorption site is the bridge-hollow site, where the two H atoms are adsorbed at the bridge and hollow sites beside a Cu atom, with an adsorption energy of –0.74 eV. In contrast, on the defective graphene-supported Cu₁₉ cluster, the favorite adsorption site is located where the two H atoms are adsorbed at hollow-hollow sites with an adsorption energy of –1.27 eV. In general, the average adsorption energy on the defective graphene-supported Cu₁₉ cluster is –1.07 eV, which is about 84% larger than that of –0.58 eV on the Cu₁₉ cluster. This indicates that the adsorption capacity is greatly enhanced for the dissociative adsorption of H₂ on the defective graphene-supported Cu₁₉ cluster. The d-band center shifts to the Fermi level, illustrating the enhanced adsorption capacity on the defective graphene-supported Cu₁₉ cluster. The integrated crystal orbital Hamilton population analysis reveals that stronger bond interactions between hydrogen atoms with their bonded Cu atoms lead to much larger adsorption energies on the defective graphene-supported Cu₁₉ cluster compared to the Cu₁₉ cluster.