11q13 amplification is a late-stage event in several cancers that is often associated with poor prognosis. Among 11q13-amplified genes, the actin assembly protein cortactin/CTTN is considered a likely candidate for direct involvement in tumor progression because of its cell motility-enhancing functions. We modulated cortactin expression in head and neck squamous cell carcinoma (HNSCC) cell lines. Cortactin expression levels directly correlated with tumor size, vascularization and cell proliferation in an orthotopic HNSCC in vivo model. In contrast, under normal in vitro culture conditions, cortactin expression levels had no effect on cell proliferation. However, cell lines in which cortactin expression was reduced by knockdown (KD) grew poorly in vitro under harsh conditions of growth factor deprivation, anchorage independence and space constraint. In contrast, overexpression of cortactin enhanced in vitro growth under the same harsh conditions. Surprisingly, defects in growth factor-independent proliferation of cortactin-KD cells were rescued by coculture with cortactin-expressing cells. As the cocultured cells are separated by permeable filters, cortactin-expressing cells must secrete growth-supporting autocrine factors to rescue the cortactin-KD cells. Overall, cortactin expression modulates multiple cellular traits that may allow survival in a tumor environment, suggesting that the frequent overexpression of cortactin in tumors is not an epiphenomenon but rather promotes tumor aggressiveness.