Background: Heterogeneity is the prominent character of TNBC and one predominant cause of treatment failure. As a key nuclear SWI/SNF protein complex subunit, ARID1A plays a critical role in tumor identity determining by manipulating gene expression. The present study aimed to investigate whether and how ARID1A determined the metastatic heterogeneity of triple-negative breast cancer. Methods: This study retrospectively collected and analysed clinical and pathological data from 258 patients diagnosed with triple-negative breast cancer at Fudan University Cancer Hospital from August 2015 to June 2016 to define the prognostic ability of ARID1A.CRISPR/Cas9 or plasmids-mediated ARID1A were used to determine the ARID1A function in regulating the metastatic character in TNBC. Cell morphology, tumor invasion, and metastasis were analyzed in terms of ARID1A status both in vitro and in vivo experiments. The molecular mechanism of ARID1A determining metastasis potential of TNBC was explored by RNA-Seq and nuclear positioning. Results: ARID1A low expression was found to have independent prognostic value for poor OS (43.9 vs 53.6 months, P=0.0001) and RFS (31.0 vs 45.5 months, P=0.0029) in TNBC patients. Both nuclear and cytoplasmic protein analysis and Immunofluorescent localization assay confirm that ARID1A recruits the Hippo pathway effector YAP into nucleus in human triple negative breast cancer cells. Next we designed the YAP truncator plasmid and confirmed by Co-Immunoprecipitation that ARID1A could competitively bind to the WW domain of YAP to form the ARID1A/YAP complex. Furthermore, down-regulation of ARID1A promoted migration and invasion both in human triple negative breast cancer cells and xenograft model through Hippo/YAP signaling axis. Conclusions: Our study provides new insights into the previously unrecognized role of ARID1A in TNBC metastasis, which might provide new therapeutic options for patients with TNBC.