Deregulation of Src kinases is associated with cancer. We previously showed that SrcDN conditional expression in MCF7 cells diminished tumorigenesis and causes tumor regression in mice. However, it remained unclear whether SrcDN affected breast cancer stem cell functionality or it reduced tumor mass. Here, we address this question by isolating an enriched population of BCSCs (ESA+-CD44+-CD24-) and the tumor-differentiated cells (ESA+-CD44+-CD24+) from MCF7-Tet-On-SrcDN. ESA+-CD44+-CD24- grew in suspension forming mammospheres, and producing tumors in nude mice, while ESA+-CD44+-CD24+ were poorly/non-tumorigenic. Doxycycline-induction of SrcDN inhibited BCSC tumorigenesis, selfrenewal, and stem-cell markers expression. SrcDN significantly inhibited SFE, and stem-cell markers expression in triple-negative breast cancer (TNBC) MDA-MB-231 and SUM159PT cells. Inducible depletion of c-Src caused similar effects in MDA-MB-231 cells. In MCF7-Tet-On-SrcDN derived mammospheres SrcDN-induction inhibited expression, and activity of hexokinase, pyruvate kinase and lactate dehydrogenase, resulting in diminished glucose consumption and lactate production, which restricted Warburg effect. Thus, c-Src functionality is important for breast cancer stem cell maintenance and renewal, tumorigenicity, and stem cell transcription factor expression, effects linked to glucose metabolism reduction.