The transcription factor STAT3 is constitutively activated in tumors of different origin but the molecular bases for STAT3 addiction of tumor cells have not yet been clearly identified. We generated knock/in mice carrying the constitutively active Stat3 allele, Stat3C, and showed that Stat3C could enhance Neu oncogenic power, triggering the production of earlier onset, more invasive mammary tumors. Tumor-derived cell lines displayed higher migration and invasion and disrupted distribution of cell-cell junction markers. The tensin family member Cten (C-Terminal Tensin-like), known to mediate EGF-induced migration and highly expressed in inflammatory breast cancer, was up-regulated in both Neu;Stat3C cells and tumors. Both Cten expression and enhanced migration were strictly dependent on Stat3, and Cten silencing normalized cell migration and rescued cell-cell contact defects. Importantly, the pro-inflammatory cytokine IL-6 could mediate Cten induction in MCF10 cells, in an exquisitely Stat3-dependent way. This model allowed us to shed some light on the oncogenic role of Stat3 in the breast, suggesting moreover a mechanism through which inflammatory signals can cooperate with EGF receptors in inflammatory breast cancer.