Breast tumors are one of the most common locations of malignant neoplasms. Every year, new aspects of the pathogenesis of breast cancer are discovered; along with genetic, hormonal and other factors, the issue of implantation and metastatic damage to the organ by tumors of other localizations is being increasingly discussed. With evolving diagnostic methods, more and more reports are emerging indicating that sometimes a breast tumor is not a primary lesion, but a site of metastasis. In this paper, the authors present a clinical case of metastasis of endometrioid adenocarcinoma of the endometrium to the breast. In this paper, the authors present a clinical case of metastasis of endometrioid adenocarcinoma of the endometrium to the mammary gland. The final diagnosis was made through analysis of a combination of morphological and immunohistochemical features. This clinical case is unique from a clinical and pathological perspective.