The gut-mammary gland axis connects the communication between the gut microbiota and the mammary gland. Studies have reported that dysbiosis in gut microbiota is involved in the pathogenesis of breast diseases such as mastitis and breast cancer. In addition, different diets, including the supplementation of probiotics or prebiotics, which are closely related to the alteration of gut microbiota, were found to affect the nutritional contents of human milk and contribute to the development or alleviation of breast diseases. These studies suggested that the gut microbiota might be a new target for the regulation of human milk components or breast diseases. In this article, we summarized recent research advances in the gut-mammary gland axis and discussed some of the effective mechanisms and pathways involved, including distal translocation of gut microbiota, circulation of metabolites across the blood-milk barrier, the delivery of immune cells and their secreted antibodies to the mammary gland. This article also provides new strategies for preventing women’s breast diseases and improving the composition of human milk through the gut-mammary axis.