Abstract: Extramedullary plasmacytoma (EMP) is a malignant neoplasm characterized by monoclonal plasma cell proliferation originating outside the bone marrow and hematopoietic tissues. The co-occurrence of multiple myeloma (MM) with mammary EMP is clinically rare. This case report describes the diagnostic and therapeutic management of a middle-aged female patient with MM and mammary EMP, accompanied by a literature review. The patient presented with persistent fever and was found to have a right breast mass via PET/CT during routine follow-up for MM. The lesion was biopsy-confirmed as mammary EMP, and the patient subsequently underwent chemotherapy. This case highlights the critical importance of differential diagnosis between breast cancer and rare hematologic metastatic tumors in early prevention of disease progression.