Oncogene-induced senescence (OIS), the proliferative arrest engaged in response to persistent oncogene activation, serves as an important tumor-suppressive barrier. We show here that finite lifespan human mammary epithelial cells (HMEC) undergo a p16/RB- and p53-independent OIS in response to oncogenic RAS that requires TGF-β signaling. Suppression of TGF-β signaling by expression of a dominant-negative TGF-β type II receptor, use of a TGF-β type I receptor inhibitor, or ectopic expression of MYC permitted continued proliferation upon RAS expression. Surprisingly, unlike fibroblasts, shRNA-mediated knockdown of ATM or CHK2 was unable to prevent RAS-mediated OIS, arguing that the DNA damage response is not required for OIS in HMEC. Abrogation of TGF-β signaling not only allowed HMEC lacking p53 to tolerate oncogenic RAS but also conferred the capacity for anchorage-independent growth. Thus, the OIS engaged after dysregulated RAS expression provides an early barrier to malignant progression and is mediated by TGF-β receptor activation in HMEC. Understanding the mechanisms that initiate and maintain OIS in epithelial cells may provide a foundation for future therapies aimed at reengaging this proliferative barrier as a cancer therapy.