Abstract: ObjectiveTo investigate the expression levels of AT-rich interactive domain-containing protein 1B（ARID1B） in triple negative breast cancer（TNBC） and its clinical significance. Methods The expression status of ARID1B in TNBC， non-TNBC and adjacent normal tissues was detected by immunohistochemistry and the correlation between the expression levels and clinicopathological characteristics and prognosis in patients with TNBC was analyzed. Results ARID1B protein expression was higher in 150 breast cancer tissues as compared to 30 adjacent normal tissues（57.5％ vs. 13.3％， P＜0.001）. The positive expression rate of ARID1B in 90 TNBC and 30 non-TNBC group were 61.1％（55／90）and 33.3％（10／30）， respectively， with a significant difference（P=0.007）. The expression levels of ARID1B were remarkably associated with age， tumor size， histological grade and Ki-67 index（P＜0.05）， while not related to lymph node metastasis， TNM stage and the expression of p53（P＞0.05） in TNBC. Particularly， compared to low-grade tumors， ARID1B displayed more bounteously expression in high-grade TNBC（P＜0.001）. The median overall survival（OS） time in TNBC patients with high ARID1B expression was 27.9 months（95％CI：25.1-30.8 months）， lower than 49.1 months（95％CI：40.2-57.9 months）of TNBC patients with low ARID1B expression（P＜0.05）. Conclusion ARID1B is highly expressed in TNBC and its expression is significantly associated with age， tumor size， histological grade， Ki-67 index and survival， which might serve as a promising biomarker for TNBC prognostic evaluation and treatment.