Abstract: Breast cancer has emerged as the leading cause of cancer incidence and mortality among women globally, with approximately 70% of cases being estrogen receptor (ER) and/or progesterone receptor (PR) positive. The primary treatment for hormone receptor (HR) positive breast cancer is endocrine therapy; however, endocrine therapy resistance remains a critical area of ongoing research. Cyclin D1 is recognized as a biomarker for tumor phenotype and disease progression, and it has been established to correlate with the onset, progression, and prognosis of breast cancer. Recently, studies have also reported the significant role of Cyclin D1 in endocrine resistance in breast cancer. This article provides an overview of the research progress on Cyclin D1 in breast cancer, with a particular emphasis on its relationship with endocrine therapy resistance, aiming to offer novel insights for the endocrine treatment of breast cancer.