Background Multiple primary malignancies of breast cancer and diffuse large B-cell lymphoma (DLBCL) are rare. Here, we report a case of advanced breast cancer and DLBCL managed with multidisciplinary therapy preceded by surgery with a successful outcome. Case presentation During a medical examination, a 71-year-old woman was diagnosed with a right breast mass, enlarged lymph nodes throughout the body, and a splenic tumor. The results of the clinical examination and imaging were suggestive of widely spread breast cancer with lymph node metastasis and malignant lymphoma with systemic metastasis. The histological evaluation of the biopsied breast tissue revealed human epidermal growth factor receptor 2 (HER2)-positive breast cancer, whereas the histological evaluation of the excised inguinal lymph node revealed DLBCL. 18 F-FDG PET/computed tomography was performed, and it was determined that both breast cancer and DLBCL were in an advanced stage. Thus, mastectomy was performed, and the axillary lymph nodes showed mixed metastasis of breast cancer and DLBCL. Soon after, the R-CHOP therapy was initiated (375-mg/m 2 rituximab, 2-mg/m 2 vincristine, 50-mg/m 2 doxorubicin, 750-mg/m 2 cyclophosphamide, and 125-mg methylprednisolone). After irradiation of the spleen, trastuzumab was administered for 1 year. Conclusions We experienced a case of combined breast cancer and DLBCL, which was difficult to treat because both were in advanced stages. Thorough staging of the malignancy and discussion by a multidisciplinary team are necessary to determine the optimal treatment strategy.