A new naphthoquinone derivative was isolated from the heartwood of the teak stem. The chemical structure of this new compound, 40,50-dihydroxy-epiisocatalponol, was determined using 1-D and 2-D nuclear magnetic resonance experiments, vibrational circular dichroism, HRMS, and optical rotation. We showed that this new naphthoquinone derivative plays a key role in the variability of decay resistance in teak wood. A high negative correlation was found between its concentration and the mass losses of the wood samples after exposure to the brown rot Antrodia sp., the fungus that is the most virulent against teak (R ¼ _0.9; r < 0.0001). In-vitro bioassays allowed us to demonstrate that 40,50-dihydroxyepiisocatalponol acted as a fungicide against Trametes versicolor (white rot) at 58 mg ml-1 (0.22 mM). Overall, our results demonstrated that the concentration of 40,50-dihydroxy-epiisocatalponol could be used as a new tool to evaluate teak wood durability.