SOX10 belongs to the SOX family of transcription factors and is an important regulator of early neural crest development. A recent study has reported SOX10 is over expressed in basal-like and triple negative subtypes of breast cancer. In the current study, we have investigated SOX10 expression in normal mammary gland, a panel of breast cancer cell lines, three public datasets: NKI (n=295), Jonsson (n=359), TCGA (n=372) and in a cohort of invasive breast cancers (n=449) sub-classified by an immunohistochemistry (IHC) surrogate panel. Our results showed that SOX10 expression is not only restricted to myoepithelial cells in the normal breast tissue but is also expressed in a sub-population of luminal cells using immunoflurosence (IF). SOX10 mRNA levels analysed in a panel of breast cancer cell lines showed SOX10 expression was exclusively observed in 2 out of 22 breast cancer cell lines (HCC1143 and MDAMB-453) which also had a triple negative (TN) phenotype. In-silico analysis showed, expression of SOX10 was enriched in the basal-like molecular subtype and the expression pattern was consistent across all three data sets analysed. Tissue microarrays (TMAs) consisting of 449 breast tumors were stained for SOX10 and other known biomarkers by IHC. Overall, 66% (26/53) triple negative breast cancers (TNBCs) and 43% of basal-like breast cancers were positive for SOX10 staining in our cohort. In positive cases, nuclear expression was observed in nearly all the tumour cells (ranging from 80% to 100%). In conclusion, SOX10 was highly expressed in triple-negative and basal-like breast cancers. Kaplan-Meier survival analysis showed over expression of SOX10 within a subset of TN and basal-like breast cancer was associated with poor prognosis (p<0.0267). Thus, our data suggests that SOX10 is a prognostic biomarker in breast cancer and further functional studies are needed to understand the biological role of SOX10 in breast cancer progression.