Breast plasmacytoma is relatively uncommon in which most of the recorded cases were related to disseminated multiple myeloma. However, many of these cases tend to be misdiagnosed with other breast lesions such as breast carcinoma. This article presents a case study on a Libyan female patient around the age of 55 who has a single breast lump, which was first diagnosed to be a malignant lesion. All the results of immunostaining for cytokeratins, GATA3, estrogen receptor, progesterone receptor, HER2, and E-cadherin were negative; hence, the possibility of a breast carcinoma was not considered. However, plasma cell tumors were indicated by the presence of CD138, MUM1, and kappa-light chain markers. In addition, the patient had multiple osteolytic bone lesions, plasma cell infiltration, a monoclonal gammopathy, and signs of renal failure, which considered to be an indication to an extra-medullary breast plasmacytoma secondary to advanced multiple myeloma. This case study emphasizes the necessity of complete histopathological and imaging evolution for proper diagnosis of breast plasmacytoma.