Background: Low tyrosine-protein phosphatase nonreceptor type 12 (PTPN12) expression may be associated with breast cancer growth, proliferation, and metastasis. However, the prognostic value of PTPN12 in breast cancer has not been clearly identified. Patients and Methods: 51 triple-negative breast cancer (TNBC) patients and 83 non-TNBC patients with a histopathology diagnosis from October 2001 to September 2006 were included in this study. Immunohistochemical staining for PTPN12 on tissue microarrays was conducted. Results: High PTPN12 expression was seen in 39.2% of TNBC and 60.2 % of non-TNBC cases. Low PTPN12 expression was associated with lymph node status (p = 0.002) and distant metastatic relapse (p = 0.002) in TNBC patients. Similarly, low PTPN12 expression in non-TNBC patients was significantly correlated with lymph node status (p = 0.002), stage (p = 0.002) and distant metastatic relapse (p = 0.039). The high PTPN12 expression group was associated with longer DFS and OS compared with low PTPN12 expression group only in TNBC cases (p = 0.005, p = 0.015), according to univariate Cox regression analysis. Conclusion: These findings provide evidence that low expression of PTPN12 is associated with worse prognosis and may be used as a potential prognostic biomarker in TNBC patients.