Mesenchymal stem cells (MSCs) are considered for potential use as an ideal vehicle to efficiently deliver therapeutic agents in treatment against cancers including melanoma. However, emerging evidence indicates that MSCs promote tumor growth and progression. Therefore, a comprehensive understanding of the role of MSCs is very important to evaluate the MSCs-based therapy in melanoma. B16 melanoma cells treated by MSC conditioned medium (CM), showed significantly enhanced migration and invasion, which was also confirmed in a lung metastasis mice model in vivo. Later, it was found that MSC CM induced an epithelial mesenchymal transition (EMT) in B16 cells. The ELISA assay showed that transforming growth factor-β (TGF-β) was secreted by MSCs and EMT was also induced by recombinant TGF-β in B16 melanoma cells, which suggests the important role of TGF-β in mediating EMT caused by MSC CM. When TGF-β signaling was inhibited by SB431542, the EMT process was significantly eliminated in vitro and in xenograft tumors in vivo. Snail, as a downstream target of TGF-β signaling and an EMT regulator, was upregulated by MSC CM and inhibited by SB431542, which confirms the key role of TGF-β signaling in EMT induced by MSC CM in B16 cells. Taken together, this study shows that MSC induces EMT in melanoma cells in a paracrine manner, which might be mediated by the TGF-β/Snail signaling pathway. Thus, caution should be exercised when considering MSCs-based therapy in melanoma and also in other cancers. Targeting TGF-β signaling and Snail could be further investigated as potential therapeutic approaches for melanoma.