Oral squamous cell carcinoma (OSCC), which is the most common malignancy of the oral cavity, is often associated with local and regional invasion. Increased expression of matrix metalloproteinase-9 (MMP-9) is correlated with invasive behavior of OSCC. Because transforming growth factor beta1 (TGF-beta1) is up-regulated in OSCC tumors, we examined the relationship between TGF-beta1 signaling and MMP-9 in human OSCC specimens. Evaluation of human specimens showed that tumors with enhanced TGF-beta1 signaling also showed increased MMP-9 expression. Because the transcription factor Snail has been determined to be a key mediator of TGF-beta1 signaling, we evaluated the role of Snail in TGF-beta1-mediated MMP-9 expression. Initially, we examined the extent to which TGF-beta1 regulated Snail expression in oral keratinocytes and in OSCC cell lines. TGF-beta1 enhanced Snail expression in a majority of the cell lines examined, with the largest induction of Snail detected in UMSCC1 cells. Interestingly, overexpression of Snail in UMSCC1 cells enhanced MMP-9 and tissue inhibitor of metalloproteinase-1 protein levels. Despite the increase in the tissue inhibitor of metalloproteinase-1 protein, there was a net increase in the pericellular proteolytic activity as shown by enhanced MMP-9-dependent Matrigel invasion. Moreover, Snail-specific siRNA blocked TGF-beta1-induced MMP-9 expression and Matrigel invasion. In addition, Snail increased Ets-1 levels and Ets-1-specific siRNA blocked both Snail- and TGF-beta1-mediated MMP-9 expression and Matrigel invasion. Thus, these data show that Snail functions as a molecular mediator of TGF-beta1-regulated MMP-9 expression by increasing Ets-1 and thereby contributing to oral cancer progression.