Primary breast lymphoma, particularly primary mucosa-associated lymphoid tissue (MALT) lymphoma, is a rare disease. We report here a case of a MALT lymphoma of the breast with high-grade transformation. An 84-year-old woman presented with a Pagetoid mass in her right breast. After a clinical diagnosis of breast cancer, T2N1M0, stage II B, she underwent mastectomy with axillary lymph node dissection. She had no history of autoimmune disease such as Hashimoto thyroiditis or Sjögren disease. Pathologically the tumor tissue was composed of small to large lymphoid cells. The large cells contained a considerable number of centroblasts, forming a sheet-like proliferation centrally. Among the small cells many small cleaved cells were present, which were more predominant in the peripheral areas. Immunohistochemistry revealed that these cells were positive for CD20 and CD79a. The tumor cells infiltrated the ductular epithelial cells, distorting the duct structures, to form lymphoepithelial lesions. Immunohistochemistry for cytokeratin and CD20 was helpful for identification. No germinal centers were formed. MALT lymphoma with high-grade transformation was diagnosed. Rituximab (anti-CD20 antibody) was prescribed as systemic treatment without chemotherapy or irradiation. After 18 months, no tumor recurrence was observed. We emphasize the importance of lymphoepithelial lesions for the diagnosis of MALT lymphoma of the breast.