Breast cancer remains a major global health concern, with persistent challenges in recurrence, treatment resistance, and therapy-related toxicity. Parallel to advancements in oncology, recent research has uncovered a compelling connection between the gut microbiota and breast cancer pathogenesis, progression, and therapeutic response. This comprehensive review synthesizes current evidence linking microbial dysbiosis to breast cancer through key mechanisms such as chronic systemic inflammation, estrogen metabolism via the estrobolome, genotoxin production, immune system modulation, and epigenetic alterations. The emerging concept of the gut-mammary axis illustrates a systemic interplay whereby gut-derived microbial metabolites and immune signals directly influence breast tissue and tumor biology. Additionally, specific microbial profiles have been shown to impact the efficacy and toxicity of chemotherapy and immune checkpoint inhibitors, opening new avenues for microbiome-targeted interventions. With growing interest in personalized nutrition, probiotics, and fecal microbiota transplantation, the gut microbiota is poised to become an integral component of precision oncology. This review highlights the translational potential of microbiome science in breast cancer prevention, prognosis, and therapy, while calling for rigorous interdisciplinary research and large-scale clinical trials to validate and integrate these findings into standard care. © 2025 Elsevier B.V., All rights reserved.