The vast assemblage of microorganisms in the human digestive tract, known as the “gut microbiota,” has gained significant attention for its role in maintaining health and contributing to disease processes, potentially influencing the risk of breast cancer (BC). Emerging research suggests that the gut microbiome may be bidirectional in the risk of breast cancer. This review explores the complex relationship between gut bacteria and BC development. The review discusses the influence of gut bacteria on estrogen metabolism, modulates immune responses, and generates various metabolites that promote cancer (foes) or inhibit cancer (friends). Gut bacteria can influence BC risk through several mechanisms. Specific bacterial groups possess enzymes like β-glucuronidase, which convert inactive estrogen conjugates into potent forms, potentially increasing their availability for cancer cell growth. Conversely, some bacteria produce short-chain fatty acids (SCFAs) with potential anticancer properties. Additionally, gut dysbiosis can trigger chronic inflammation, disrupting immune function and promoting tumor development. The gut microbiome emerges as a potential BC prevention and treatment strategy target. There is a need to focus future research on identifying specific microbial signatures associated with increased or decreased risk in BC and exploring potential therapeutic interventions to manipulate the gut microbiome for a beneficial effect.