Breast cancer (BC) impacts 2.3 million women each year, making it the most frequent cancer diagnosed among the female population. An unexpected link has been discovered between BC and alterations in the mammary and gut microbiota, suggesting their possible role in BC development, prevention and management. Studies suggest a distinct microbiome in healthy breast tissue compared to BC tissue. The healthy breast tissue has been found to be mostly enriched with bacteria of the phyla and . However, certain bacteria are more abundant in cancerous tissues compared to adjacent non‑cancerous tissues in BC women or compared to the breast tissues of healthy women. On the other hand, bacteria such as spp. are increased in the breast tissues of healthy women compared to the cancerous tissues of BC women and may therefore have potential protective effects against BC. Additionally, preliminary studies propose that the mammary microbiota is distinct in the different subtypes of BC, proposing a specific role of microbes in the development of BC and suggesting their possible use as biomarkers. Similarly, dysbiosis in the gut microbiota has been further linked to BC since certain gut bacteria can alter the production of beneficial metabolites and disrupt estrogen metabolism in the gut. While still at its infancy, such unexpected links between breast and gut microbiota and BC propose possible alternatives with regards to the prevention but also management of BC such as through the use of probiotics. The current review is focused on evaluating the recent evidence regarding the association between mammary and gut microbiota and BC and discusses the most important bacteria involved.