Primary rectal adenocarcinoma metastatic to the breast is an exceedingly rare event. Its management differs from that of primary breast cancer, as illustrated by this case. A 63-year-old woman presented with a breast lump 30 months after abdominoperineal resection for rectal adenocarcinoma, stage T₃N₁M₀ (stage III), followed by standard postoperative radiochemotherapy. The patient underwent a mammography and ultrasonography. A CT scan of the abdomen showed metastatic disease. An excisional biopsy of the breast lump was performed; morphological features were identical to the original rectal cancer. Immunohistochemical results were negative for estrogen and progesterone receptors and gross cystic disease fluid protein-15, and intensity positive for cytokeratin 20 and carcinoembryonic antigen. The patient died after treatment with palliative chemotherapy. Metastatic disease from rectal carcinoma to the breast is a marker for disseminated metastatic spread with poor prognosis.