Xanthones are the main active constituents isolated from Garcinia mangostana L. (Hypericaceae). Among these, α-mangostin (1) is reported as the principal component of the species, with remarkable pharmacological activities. In this work, a reverse-phase high-performance liquid chromatography (HPLC) method was developed as a suitable stability-indicating procedure for the quantification of 1 in the presence of degradation products. Chromatographic analysis was performed on a Purospher STAR C18 column with a mobile phase containing 0.1% trifluoroacetic acid in water and ACN (20:80) at a flow rate of 1.0 mL/min. The method was linear over the concentration range of 10 to 100 μg/mL. The limits of detection and quantification were 0.13 and 0.4 μg/mL, respectively. Stability studies were carried out at temperature/relative humidity of 30°C/60% and 40°C/75% during 6 months; and no significant changes were observed. According to the global results, a simple HPLC method was developed and fully validated for quantifying 1 in mangosteen dietary supplements.