A case of synchronous carcinoma of the accessory mammary gland and primary breast lymphoma with subsequent rectal carcinoma has not been reported previously. We present a very rare case of primary non-Hodgkin lymphoma of the left breast diagnosed simultaneously with invasive lobular carcinoma of the left axillary accessory mammary gland and rectal adenocarcinoma. An 82-year-old Japanese woman presented with two palpable masses on the left chest wall. She was given a diagnosis of suspected breast malignant tumor and axillary accessory mammary gland. She underwent excision of the axillary accessory mammary gland and left mastectomy with axillary lymph node dissection. Histopathological examination revealed diffuse large B-cell lymphoma of the breast and invasive lobular carcinoma of the axillary accessory mammary gland with lymph nodes metastasis. Three months after the surgery, primary rectal adenocarcinoma was also detected by F-18 fluorodeoxyglucose positron emission tomography. Hartmann's operation was performed, since which time the patient has been doing well.