Abstract: As a highly heterogeneous disease, breast cancer ranks first in new cases and deaths of female malignant tumors. In recent years, the incidence of breast cancer increases and tends to be younger. Early-onset breast cancer (<40 years old) is mostly associated with adverse biological characteristics and poor prognosis. Although immunotherapy and targeted therapy have advances in the treatment of breast cancer, a major challenge of neoplastic resistance to systemic therapy remains. Therefore, early diagnostic markers and potential therapeutic targets with high specificity and sensitivity must be identified. miR-21 affects the occurrence and development of breast cancer by targeting related genes and regulating related signaling pathways (PTEN/PI3K/Akt and NF-κB/ miR-21-5p/PDCD4 signaling pathways). This paper reviews the mechanism and progress of miR-21 in breast cancer.