Proteasome activator subunit 3 (PSME3) plays a key role in breast cancer by regulating the cell cycle. However, its role in other pathogenesis-related features of breast cancer is unclear. In this study, we found that overexpression of PSME3 induced the epithelial–mesenchymal transition and contributed to induce the expression of cancer stem cell markers of the MDA-MB-231 cell line, thus increasing the migration, and invasion of the cells. Moreover, overexpression of PSME3 reduced the chemotaxis of CD8+ T cells and induced the apoptosis of T cells in vitro. Furthermore, PSME3 knockdown increased the number of CD8+ T cells in vivo and reduced the subcutaneous tumor growth rate. These findings revealed that PSME3 induces epithelial–mesenchymal transition with inducing the expression of CSC markers and influencing the tumor immune microenvironment in breast cancer.