Breast tumor interleukin-6 (IL-6) levels increase with tumor grade, and elevated serum IL-6 correlates with poor breast cancer patient survival. Epithelial-mesenchymal transition (EMT) phenotypes such as impaired E-cadherin expression or aberrant Vimentin induction are associated with enhanced metastasis and unfavorable clinical outcome in breast cancer. Despite this fact, few tumor microenvironment-derived extracellular signaling factors capable of provoking such a phenotypic transition have been identified. In this study, we showed that IL-6 promoted E-cadherin repression among a panel of estrogen receptor-alpha-positive human breast cancer cells. Furthermore, ectopic stable IL-6 expressing MCF-7 breast adenocarcinoma cells (MCF-7(IL-6)) exhibited an EMT phenotype characterized by impaired E-cadherin expression and induction of Vimentin, N-cadherin, Snail and Twist. MCF-7(IL-6) cells formed xenograft tumors that displayed loss of E-cadherin, robust Vimentin induction, increased proliferative indices, advanced tumor grade and undifferentiated histology. Finally, we showed aberrant IL-6 production and STAT3 activation in MCF-7 cells that constitutively express Twist, a metastatic regulator and direct transcriptional repressor of E-cadherin. To our knowledge, this is the first study that shows IL-6 as an inducer of an EMT phenotype in breast cancer cells and implicates its potential to promote breast cancer metastasis.