This study first investigates the anti-metastatic effect of alpha-mangostin on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) expressions in human breast adenocarcinoma cells, MCF-7. First, the result demonstrated alpha-mangostin could inhibit TPA-induced abilities of the adhesion, invasion, and migration by cell-matrix adhesion assay and Boyden chamber assay. Data also showed alpha-mangostin could inhibit the activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) involved in the downregulation the enzyme activities, protein, and messenger RNA levels of MMP-2 and MMP-9 induced by TPA. Next, alpha-mangostin also strongly inhibited TPA-induced degradation of inhibitor of kappaBalpha (IkappaBalpha) and the nuclear levels of nuclear factor kappa B (NF-kappaB), c-Fos, and c-Jun. Also, a dose-dependent inhibition on the binding abilities of NF-kappaB and activator protein-1 (AP-1) by alpha-mangostin treatment was further observed. Further, the treatment of specific inhibitor for ERK (U0126) to MCF-7 cells could inhibit TPA-induced MMP-2 and MMP-9 expressions along with an inhibition on cell invasion and migration. Presented data reveal that alpha-mangostin is a novel, effective, antimetastatic agent that functions by downregulating MMP-2 and MMP-9 gene expressions.