Objective To explore the value of the prognostic nutritional index (PNI) in predicting the prognosis of triple-negative breast cancer patients. Methods The clinical data of 93 triple-negative breast cancer patients were retrospectively analyzed. The receiver operating characteristic (ROC) curve was used to analyze the value of PNI in predicting the prognosis of triple-negative breast cancer patients, the binomial logistic regression model was used to investigate influencing factors for the prognosis of triple-negative breast cancer patients, and a nomogram model for predicting the prognosis of triple-negative breast cancer patients was constructed. Results The ROC curve analysis showed that the area under the ROC curve of PNI was 0.717 when predicting the prognosis of triple-negative breast cancer patients, and the optimal cut-off point was 46.5, corresponding to which the sensitivity and specificity were 74.3% and 69.0%, respectively (P<0.05). The results of binomial logistic regression analysis showed that tumor length, differentiation degree, brain metastasis, and PNI were independent influencing factors for the prognosis of triple-negative breast cancer patients, among which long tumors and the presence of brain metastasis were independent risk factors and medium and high differentiation degrees and a high PNI were independent protective factors (all P<0.05). The calibration curve showed that the predicted values of the nomogram prediction model for the prognosis of triple-negative breast cancer patients, which was constructed based on the above factors, were highly consistent with the actual values. Conclusion PNI may be used as a predictor of prognosis in triple-negative breast cancer patients. Long tumors and the presence of brain metastasis are independent risk factors, while the medium and high differentiation degree and a high PNI are independent protective factors for the prognosis of triple-negative breast cancer patients, the nomogram prediction model, which was constructed based on the above factors, has high clinical application value.