Tumorigenesis is a multi-step process involving dysregulated cell growth and metastasis. Considerable evidence implicates a mitogenic action of estrogen in early ovarian carcinogenesis. In contrast, its influence in the metastatic cascade of ovarian tumor cells remains obscure. In the present study, we showed that 17-estradiol (E2) increased the metastatic potential of human epithelial ovarian cancer cell lines. E2 treatment led to clear morphological changes characteristic of epithelial-mesenchymal transition (EMT) and an enhanced cell migratory propensity. These morphologic and functional alterations were associated with changes in the abundance of EMT-related genes. Upon E2 stimulation, expression and promoter activity of the epithelial marker E-cadherin was strikingly suppressed, whereas EMT-associated transcription factors Snail and Slug were significantly up-regulated. This up-regulation was attributed to the increase in gene transcription activated by E2. Depletion of endogenous Snail or Slug using small interfering RNA (siRNA) attenuated E2-mediated decrease in E-cadherin. In addition, E2-induced cell migration was also neutralized by the siRNAs, implicating that both transcription factors are indispensable for the pro-metastatic actions of E2. More importantly, by using selective ER agonists, forced expression, and siRNA approaches, we identified that E2 triggered the metastatic behaviors exclusively through an ER-dependent pathway. We also showed that ER had an opposing action on ER since the presence of ER completely inhibited the EMT and down-regulation of E-cadherin induced by ER. Collectively, this study provides compelling argument that estrogen can potentiate tumor progression by EMT induction, and highlights the crucial role of ER in ovarian tumorigenesis.