<strong>Objective</strong>. Using molecular simulation, we studied the influence of Mg2+ ions on the binding mode of HTLV-I Integrase (IN) catalytic domain (modeled by homology) with the 3,5- Dicaffeoylquinic Acid (DCQA). HTLV-I Integrase homology model was built using template-like crystallographic data of the IN catalytic domain solved for Avian Sarcoma Virus (VSA, pdb: 1VSD). <strong>Materials and</strong> <strong>methods</strong>. In order to analyze the role of Mg2+ in the interaction or coupling between 3,5-DCQA and Integrase, three models were created: i) in the absence of Mg2+ ions, ii) with a Mg2+ ion coordinated at Asp15 and Asp72 and iii) model with two Mg2+ ions coordinated at Asp15-Asp72 and Asp72-Glu108. Coupling force and binding free energy between 3,5-DCQA and HTLV-I IN were assessed in the three models. <strong>Results</strong>. The lowest docking score and free energy binding were obtained for the second model. Mg2+ ion strongly affected the coupling of the inhibitor 3,5-DCQA with HTLV-I catalytic domain of Integrase, thus revealing a strong interaction in the ligand-protein complex regardless of the ligand-catalytic interaction sites for all three models. <strong>Conclusion</strong>. Altogether, these results strengthen the hypothesis that the presence of one Mg2+ ion could enhance the interaction in the complex by decreasing free energy, therefore increasing the affinity. Moreover, we propose 3, 5-DCQA as an important pharmacophore in the rational design of new antiretroviral drugs.<br /><br /><strong>Key words</strong>: 3,5 -Dicaffeoylquinic Acid, Human T-Lymphotropic Type I (HTLV-1), Integrase (IN), Homology Model, Molecular Docking, Binding Free Energy, Mg2+ Ions.