Cancer-associated fibroblasts (CAF) are crucial in regulating cancer progression, yet the molecular mechanisms that determine the tumor-regulating function of CAF are poorly understood. Here, we uncover the Notch1 pathway as a molecular determinant that controls the regulatory role of CAF in melanoma growth, invasion and metastasis, and can be manipulated to reprogram and convert CAF to act as tumor suppressors. The Notch1-determined tumor-regulating function is in part mediated by Wnt-induced secreted protein 1 (WISP1). These findings reveal the Notch1—WISP1 axis as a crucial molecular determinant in governing stromal regulation of tumor progression, and establish this axis as a potential therapeutic target Total RNA obtained from MSCD-SF transduced with Cre-GFP/lentivirus in vitro compared to MSCD-SF transduced with GFP/lentivirus. Samples are duplicates.