miR205 has a dual activity, as tumor suppressor and as oncogene. Here we analyzed the impact of miR205 ectopic expression in the initial tumorigenic processes of SUM159PT, a triple negative breast cancer cell line with low endogenous levels of miR205. In SUM159PT, miR205 inhibited expression of its targets VEGFA, ErbB3, Zeb1, Fyn and Lyn A/B; it reduced cell proliferation, and Myc/cyclin D1 levels, while increased p27kip1 expression. miR205 abolished anchorage-independent growth, inhibited migration and invasion, Src-kinases/Stat3 axis activation, and levels of secreted MMP9. miR205 also reduced expression of CD44 and TAZ, E2A.E12, Twist, Snail1 and CK5, associated with epithelial-mesenchymal transition (EMT). Importantly, we show that miR205 inhibited SUM159PT cancer-stem cell renewal, expression in mammospheres of CD44 and ALDH1 stem-cell markers, TAZ, and E2A.E12. All these effects of miR205 were reverted by Anti-miR205 co-expression, demonstrating its specificity. Thus, all these results strongly suggest that ectopic expression of miR205 in SUM159PT affected several parameters associated with initial steps of tumorigenesis.