Objective: To investigate the expression and clinicopathological significance of high mobility group box protein B1 (HMGB1) protein in breast cancer. Methods: The expression of HMGB1 protein in 26 normal breast tissues and 417 invasive breast cancer tissues diagnosed at Dongyang People′s Hospital, Zhejiang Province from 2016 to 2018 were detected by immunohistochemical EnVision method. The relationship between nuclear and cytoplasmic HMGB1 protein expression and clinicopathologic features of breast cancer patients were analyzed. Results: The nuclear and cytoplasmic expression of HMGB1 protein was 80.8% (337/417) and 16.8% (70/417) respectively in breast cancer, and was 46.2%(12/26) and 0(0/26) respectively in normal breast tissue. Both nuclear and cytoplasmic expression of HMGB1 protein in breast cancer were significantly higher than normal breast tissue (P<0.001, P=0.046, respectively). The nuclear expression of HMGB1 protein was also higher in high grade, estrogen receptor (ER) negative, progesterone receptor (PR) negative (P=0.006, P=0.004, P<0.001, respectively); whereas the cytoplasmic expression of HMGB1 protein was also higher in high grade, estrogen receptor (ER) negative, progesterone receptor (PR) negative (P<0.001 in all) breast cancers. Multivariate logistic regression model showed that nuclear HMGB1 expression correlated with histologic grade (OR=2.188, 95%CI=1.078-4.443, P=0.030), while cytoplasmic HMGB1 expression correlated with histologic grade (OR=3.031, 95%CI=1.600-5.742, P=0.001), ER (OR=0.129, 95%CI=0.034-0.494, P=0.003) and TNM staging (OR=3.820, 95%CI=1.042-14.001, P=0.043). Multivariate analysis of Cox proportional hazard model showed that nuclear HMGB1 expression was an independent risk factor for the overall survival of breast cancer patients (HR=0.366, 95%CI=0.138-0.972, P=0.044). Conclusion Nuclear and cytoplasmic HMGB1 proteins are related to multiple poor prognostic factors in breast cancer, and may be a potential biomarker for breast cancer treatment.