ABSTRACT Background This article presents an extensive review of the advancements in research concerning extrachromosomal circular DNA (ecDNA) within the context of breast cancer. As a distinct form of DNA, ecDNA is critically involved in the initiation, progression, diagnosis, and treatment of breast cancer. Methods The article provides a comprehensive analysis of the mechanisms underlying the formation of ecDNA, highlighting factors such as aberrant DNA damage repair and chromosomal rearrangements. It further examines the biological roles of ecDNA in augmenting oncogene expression, fostering tumor heterogeneity, and facilitating immune evasion. Results Epidemiological studies indicate significant variability in the distribution of ecDNA across different breast cancer subtypes, with a notable association with invasive subtypes, such as triple‐negative breast cancer. The presence of ecDNA is linked to poor prognosis and treatment resistance in patients. Current detection technologies for ecDNA include molecular biology and imaging techniques, and its detectability in plasma underscores its potential as a biomarker for liquid biopsy. Conclusions The development of ecDNA‐targeted therapies and their integration with immunotherapy strategies offers promising new avenues for breast cancer treatment. Despite challenges such as incomplete elucidation of mechanisms, standardization of detection methods, and ethical considerations, ecDNA remains a valuable biomarker and therapeutic target in breast cancer. Future research is anticipated to advance its clinical transformation and application in individualized treatment.