<p>Recent studies have established the gut microbiome as a crucial player in breast cancer diagnosis, progression, and treatment. Distinct microbial patterns have shown promise as non-invasive diagnostic and prognostic biomarkers, supporting patient stratification and risk assessment based on microbiota composition. The gut microbiome also modulates estrogen metabolism, influencing the risk of hormone receptor-positive breast cancer, while dysbiosis can promote chronic inflammation and tumor expansion. Moreover, accumulating evidence demonstrates that gut bacteria can alter responses to chemotherapy and immunotherapy, suggesting that microbiota modulation may enhance treatment efficacy. With the advent of omics technologies and machine learning, intricate host–microbe interactions are being decoded, revealing new molecular targets and therapeutic opportunities. Importantly, early clinical and interventional studies using probiotics, prebiotics, and fecal microbiota transplantation (FMT) are being explored to restore microbial balance, mitigate therapy-related side effects, and improve antitumor immunity in breast cancer patients. Together, these advances underscore the translational potential of microbiome research, paving the way for microbiota-guided diagnostic, prognostic, and therapeutic strategies in personalized breast cancer management.</p>