Abstract: The G protein-coupled estrogen receptor(GPER or GPR30) mediates estrogenic signal in different cell types, especially in the hormone-sensitive tumors including breast cancer and endometrial carcinoma. Estrogens and anti-estrogens promote the activation of the downstream signaling pathways and the expression of target genes through activating GPER, which involves in the malignant biological behaviors of breast cancer cells, such as cell proliferation, migration, invasion and tamoxifen resistance, etc. It has been reported that the key target for GPER signal to enhance the biological effects of its downstream is the transactivation of epidermal growth factor receptor(EGFR). Moreover, GPER has also been proposed as a candidate biomarker in the prognosis prediction of triple-negative breast cancer, opening broad prospects for a more comprehensive assessment and treatment of breast tumor patients.