Apigenin has special interest for the development of chemopreventive agents against cancer because it is a widely distributed plant flavonoid that has antitumor properties. In this study, we investigated the apigenin effects on the protease-mediated invasiveness in human metastatic cancer cell lines Caski, SK-Hep1, and MDA-231. We found that apigenin markedly inhibits the phorbol-12-myristate-13-acetate (PMA)-induced increase in MMP-9 expression and activity in several cancer cell lines. These effects of apigenin are dose-dependent and correlate with the suppression of MMP-9 mRNA expression levels. PMA caused about a 5-fold induction in MMP-9 promoter activity, which was also suppressed by apigenin treatment in Caski cells. We found that apigenin could inhibit PMA-induced phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which was involved in the down-regulation of the expression of matrix metalloproteinase-9 (MMP-9) at mRNA levels. Furthermore, the treatment of inhibitors specific for p38 MAPK (SB203580) to Caski cells caused the reduction of MMP-9 expression. Restoration of p38 expression partly increased PMA-induced MMP-9 secretion blocked by apigenin treatment in Caski cells. These results showed apigenin might inhibit the invasion and migration abilities of Caski cells by reducing the MMP-9 expression through suppressing the p38 MAPK signaling pathway. These findings indicate that apigenin might be a useful strategy for controlling metastasis and the invasiveness of tumors.