Mangosteen (Garcinia mangostana, Clusiaceae) is called “queen of fruit” in Southeast Asia. In the current study, three dimeric xanthones, garcinoxanthones A–C, and four monomeric xanthones, garcinoxanthones D–G, together with 18 known xanthones, were isolated from the pericarps of G. mangostana, collected in Thailand. The structures of garcinoxanthones A–G were elucidated by analysis of their 1D and 2D NMR and other spectroscopic data, and their absolute configurations were determined by the CD spectra. All seven compounds were tested for nitric oxide (NO) inhibitory activity on lipopolysaccharide (LPS)-stimulated RAW264.7 cells. Garcinoxanthones B and C significantly inhibited NO production with IC values of 11.3 ± 1.7 and 18.0 ± 1.8 μM, respectively, which were comparable with the positive control indomethacin (IC 3.9 ± 0.3 μM). Moreover, garcinoxanthone B suppressed inducible NO synthase expression in a dose-dependent manner. These results reveal the presence of rare dimeric xanthones in G. mangostana and their NO inhibitory effect on LPS-stimulated murine macrophage cells.