The purpose of this study was to investigate whether pre-operative prognostic nutritional index (PNI), an indicator of nutritional and immunological status, has an impact on the long-term outcomes in triple-negative breast cancer (TNBC) patients. This retrospective study reviewed the medical records of 382 TNBC patients who had suffered from mastectomy. Pre-operative PNI was calculated as 10 × serum albumin (g/dl) + 0.005 × total lymphocyte count (per mm(3)). The receiver operating characteristic (ROC) curve analysis was performed to determine the optimal cutoff value of PNI. The correlations of PNI value with clinicopathological features were analyzed and the univariate and multivariate analysis were applied to identify the prognostic factors. The results showed that pre-operative PNI value was significantly related to advanced tumor status such as N stage (p = 0.011), T stage (p = 0.015), and recurrence incidents (p = 0.001). Survival analysis identified PNI as an independent prognostic factor for TNBC. Patients with higher PNI value had better 5-year disease-free survival (DFS) and 5-year overall survival (OS) than those with lower PNI value (DFS, p = 0.007; OS, p = 0.011). Taken together, our results suggest that the pre-operative PNI can be used as a simple and useful marker for predicting the long-term outcomes of TNBC patients.