A series of xanthone derivatives, isolated from Calophyllum teysmannii var. inophylloide, have been evaluated for their binding affinity to transthyretin. Transthyretin is a plasma protein involved in the transport of thyroxine (T4) and also implicated in amyloid diseases. Using competition-binding studies with the protein natural ligand T4, we have identified one prenylated xanthone with a very strong affinity to transthyretin. Molecular docking simulations show that the flexible tail of the prenylated xanthone could allow favorable molecular interactions. Since this xanthone may play a role in the thyroxine metabolism and/or over the pathogenic process associated with the amyloid disease, these results may be explored for the design of new ligands.