BACKGROUND: The gastrointestinal microbiota can modulate systemic estrogens, potentially influencing estrogen-induced breast neoplasia development. This study aimed to assess alterations in the gut microbiota in breast cancer patients. METHODS: A search strategy was developed using the terms: "Microbiota," "Gastrointestinal Microbiome," "Breast Cancer," and synonyms. Ten observational studies were included. RESULTS: The total sample was 1730 women (929 cases and 801 controls). The meta-analysis of alpha diversity, assessed by the Shannon index, displayed that in the breast cancer group, the diversity of the gut microbiota was reduced compared to controls, with a standardized mean difference (SMD) of - 0.34 (95% CI - 0.59, - 0.10, I CONCLUSIONS: The study found a significant difference in gut microbiota diversity between women with breast cancer and controls, supporting the growing evidence that the gut microbiota may play a role in mammary carcinogenesis.