Alpha-mangostin, a component of Garcinia mangostana Linn, is a xanthone derivative shown to have antioxidant and anticarcinogen properties. In this study, we first report the antimetastatic effect of alpha-mangostin in the human prostate carcinoma cell line PC-3. The results show that alpha-mangostin exhibited an inhibitory effect on the abilities of adhesion, migration, and invasion by cell-matrix adhesion assay, wound healing assay, and Boyden chamber assay. In the cancer cell metastasis process, matrix degrading proteinases are required. Results from zymography showed that alpha-mangostin treatment could decrease the expressions of matrix metalloproteinase-2 (MMP-2), matrix metalloproteinase-9 (MMP-9), and urokinase-plasminogen activator (u-PA) in a concentration-dependent manner. Moreover, alpha-mangostin also exerted an inhibitory effect on phosphorylation of c-Jun N-terminal kinase 1 and 2 (JNK1/2) and inhibition of activation of nuclear factor kappa B (NF-kappaB), c-Fos, and c-Jun. Furthermore, the treatment of inhibitors specific for JNK (SP600125) to PC-3 cells could result in a reduced expression of MMP-2, MMP-9, and u-PA. These results demonstrated that alpha-mangostin could mediate PC-3 cells metastasis by reduction of MMP-2, MMP-9, and u-PA expression through the suppression of the JNK1/2 signaling pathway and inhibition of NF-kappaB and AP-1 binding activity. These findings proved that alpha-mangostin might be offered further application as an antimetastatic agent.