Mammalian NUMB is alternatively spliced generating four isoforms NUMB1-NUMB4 that can function as tumor suppressors. NUMB1-NUMB4 proteins, which normally determine how different cell types develop, are reduced in 21% of primary breast tumors. Our previous work has, however, indicated that two novel NUMB isoforms, NUMB5 and NUMB6 have the pro-oncogenic functions. Herein, we address a novel function of human NUMB isoform 6 (NUMB6) in promoting cancer cell migration and invasion. We found that NUMB6 induced expression of embryonic transcription factor Slug, which in turn actively repressed E-cadherin, prompting cells to undergo epithelial-mesenchymal transition (EMT). Low-metastatic breast cancer cells DB-7 stably expressing NUMB6, lost their epithelial phenotype, exhibited migratory and pro-invasive behavior, and ultimately elevated expression of mesenchymal markers. Among these markers, increased vimentin, β-catenin, and fibronectin expression elicited metalloproteinase 9 (MMP9) production. Our results revealed that NUMB6-DB-7 cells have significantly increased level of Akt1 and Akt2 phosphorylation. Therefore, antagonizing Akt signaling using a chemical inhibitor LY294002, we found that NUMB6-induced Slug expression was reduced, and ultimately accompanied with decreased cell migration and invasion. In summary, this study identified a novel molecular determinant of breast cancer progression, uncovering a potential oncogenic role for the NUMB6 protein in cancer cell migration and invasion, coupled to the maintenance of mesenchymal-like cells. J. Cell. Biochem. 118: 237-251, 2017. © 2016 Wiley Periodicals, Inc.