Cancer/testis antigen (CTA) SSX2, which is silent in most normal adult tissues and expressed in various malignant tumors, has been identified for decades. Expression of SSX in tumors has been associated with advanced stages and worse patient prognosis. However, little is known about its role in breast cancer. The SSX2 expression plasmid constructed was stably transfected into the breast cancer cell line MCF-7. The influence of SSX2 on MCF-7 cells was assessed using MTT assay, flow cytometry, transwell invasion assay and in vivo tumorigenicity assay. A comparative proteomic approach was performed to identify and clarify the underlying molecular mechanisms. SSX2 expression was more pronounced in ERα-negative breast cancer cells compared with the positive ones. Overexpression of SSX2 induced cell growth and prompted cell invasion. Both ERα and E-cadherin expression were suppressed in the SSX2 overexpressing MCF-7 cells. Eleven known proteins were identified with significant differential expression. Among these, five were decreased, while other six were increased in the SSX2 overexpressing MCF-7 cells. These results suggested SSX2 may enhance invasiveness in MCF-7 cells both in vivo and in vitro. The regulation of ERα signaling by target proteins of SSX2 may explain the metastatic potential of breast cancer cells.