Breast cancer is the most common malignancy in women and is considered as a complex and heterogeneous disease. The identification of novel biomarkers for early detection of breast cancer and prediction of disease outcome is urgently required. MicroRNAs (miRNAs) are small non-coding endogenous RNA molecules that act to regulate gene expression and play vital roles in many crucial processes. Recent evidence demonstrates that miRNAs could emerge as revolutionary sources of biomarkers for cancer diagnosis and prognosis. miR-21 is one of the most commonly observed aberrant miRNAs in a variety of cancers including breast cancer. Emerging studies show that miR-21 could be measured stably and easily in tumor tissues, formalin-fixed paraffin-embedded tissues and blood circulation. In this review, we will summarize the current evidence of miR-21 as a promising biomarker for diagnosis and prognosis in breast cancer. We will also discuss the issues and challenges of miR-21 as a potential biomarker in future clinical applications.