Background: Triple-negative breast cancer (TNBC), characterized by the absence of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). The treatment of patients with TNBC has been challenging due to the absence of these molecular targets and the heterogeneity of the disease. Therefore a better understanding of the molecular and histopathological features of TNBC and its heterogeniety is important for the development of a new therapeutic strategy and to improve the prognosis of TNBC. Recent studies suggest that there are links between TNBC and the epithelial-mesenchymal transition (EMT). To identify prognostic biomarkers and novel therapeutic targets, vimentin one of the most major factors associated with EMT was investigated in TNBC. Methods: Sporadic invasive ductal carcinoma specimens were obtained from 659 Japanese patients, and 90 (14%) cases were diagnosed as TNBC. The vimentin mRNA and protein expression levels were evaluated by quantitative RT-PCR and immunohistochemistry. Results: The mRNA expression of vimentin was significantly upregulated in the basal type breast cancer cell line. Immunohistochemically, the vimentin expression was significantly higher (p=0.0042) in TNBC compared to the other subtypes. Vimentin expression was associated with a younger age (p=0.016), high nuclear grade (p=0.023) and high Ki67 expression (p<0.0001), and a poorer prognosis in terms of both the recurrence-free survival (RFS) (p=0.0058) and overall survival (OS) (p=0.013) in TNBC patients. A multivariate analysis showed that vimentin expression was an independent prognostic factor for the RFS (p=0.043). Vimentin expression was also associated with a significantly shorter RFS (p=0.021) and OS (p=0.017) in patients with basal-like breast cancer (BLBC). Conclusions: The elevated expression of vimentin contributes to the aggressive phenotype and poor prognosis in TNBC. Vimentin expression might be useful as a biomarker for the prognosis of TNBC.