The expression of podoplanin, a small mucin-like protein, is upregulated in the invasive front of a number of human carcinomas. We have investigated podoplanin function in cultured human breast cancer cells, in a mouse model of pancreatic β cell carcinogenesis, and in human cancer biopsies. Our results indicate that podoplanin promotes tumor cell invasion in vitro and in vivo. Notably, the expression and subcellular localization of epithelial markers are unaltered, and mesenchymal markers are not induced in invasive podoplanin-expressing tumor cells. Rather, podoplanin induces collective cell migration by filopodia formation via the downregulation of the activities of small Rho family GTPases. In conclusion, podoplanin induces an alternative pathway of tumor cell invasion in the absence of epithelial-mesenchymal transition (EMT).