Metadherin (MTDH), also known as astrocyte elevated gene-1 (AEG-1), is an oncoprotein closely related to the development of breast cancer. However, few studies have been done on the expression and clinical significance of MTDH in human epidermal growth factor receptor-2 (HER-2) positive breast cancer patients. This study aimed to investigate the expression of MTDH in locally advanced HER-2 positive breast cancer, and evaluate the clinical significance of MTDH in predicting the prognosis of patients with HER-2 positive advanced breast cancer who received the neoadjuvant chemotherapy plus trastuzumab. In 144 HER-2 positive breast cancer tissues, 79 cases showed high expression of MTDH and 65 cases showed low expression. The expression of MTDH in locally advanced HER-2 positive breast cancer tissues was correlated with TNM stage, lymph node metastasis, Miller-Payne (MP) grade, and pathologic complete response (pCR) status (P<.05), but was not correlated with patient age, estrogen receptor (ER) expression level, progesterone receptor (PR) expression level, and Ki-67 expression level (P>.05). Kaplan-Meier univariate analysis revealed a negative correlation between MTDH expression and the disease-free survival (DFS) and overall survival (OS) in the post-operative patients with locally advanced HER-2 positive breast cancer (log rank test: P<.001). By using the COX proportional hazard regression model, it was found that MTDH expression, TNM stage, lymph node metastasis, and Ki-67 expression were closely related to DFS in patients. The hazard ratio (HR) of high MTDH expression was 1.816 (95% CI: 1.165-2.829). In addition, MTDH expression, TNM stage, and lymph node metastasis were also closely related to the OS of patient. The HR of the high expression of MTDH was 2.512 (95% CI: 1.472-4.286). The expression of MTDH in tumor tissues of patients with HER2-positive locally advanced breast cancer was significantly elevated, which was related to the poor pathological features. High MTDH expression was closely correlated with poor prognosis of patients and was an important factor affecting tumor progression.