A 54-year-old woman with pseudomyxoma peritonei from an appendiceal signet ring cell carcinoma was referred to our hospital. Right massive effusion with cytology-proven malignant cells was controlled with thoracentesis. Pathological study after intraperitoneal (IP) chemotherapy, hyperthermic IP chemotherapy, and cytoreductive surgery showed no malignant cells in the abdomen except for the appendix and greater omentum. Although the patient noticed a right breast mass, mammography (MMG) showed no abnormality. Ultrasonography showed right breast masses consisting of hypo- and hyper-echoic areas without clear tumor margins. Magnetic resonance imaging (MRI) with contrast medium showed multiple lesions showing persistent enhancement pattern. Pathological study of the vacuum-assisted biopsy specimen showed signet ring cells growing in diffuse, trabecular, and linear fashions, leading to the diagnosis of metastatic breast tumors from the appendiceal signet ring cell carcinoma. Positron emission tomography (PET)/computed tomography (CT) showed no fluorodeoxyglucose uptake in the breasts. The patient was treated with simple mastectomy for local control. Pathological study of the resected breast showed predominant localization of the metastatic breast tumors deep in the mammary gland and lymphovascular invasion. Metastatic breast tumors from appendiceal signet ring cell carcinomas are extremely rare and can sometimes be difficult to detect with MMG and PET/CT. General surgeon should note that appendiceal signet ring cell carcinomas can metastasize to the breast, presumably through lymphatic permeation from malignant pleural effusion, without abnormal MMG and PET/CT findings.