INTRODUCTION: Extramedullary plasmacytoma of the breast is an uncommon neoplasm, occurring either as a solitary tumour or as an evidence of disseminated multiple myeloma. 1 Plasma cells represent terminally differentiated B-cells. Their neoplasms are characterized by secretion of a single homogenous immunoglobulin product known as the monoclonal or "M" component, which can be detected by serum and urine protein electrophoresis. 2 MATERIAL AND METHODS: We describe here a case of breast plasmacytoma in a 56 year old male patient who was a known case of multiple myeloma. Diagnosis was made on the histopathological examination, Serum protein electrophoresis, X-ray finding and positive immunohistochemical staining with Lambda Light chain - Positive and CD138-Positive, In Pathology department of Mahatma Gandhi Medical College & Hospital, Jaipur. DISCUSSION: Plasmacytoma in breast is rare Extramedullary Plasmacytoma usually occur in the head and neck area in 90% of cases, for example, the upper respiratory tract, nasal cavity, paranasal sinuses, oropharynx and salivary glands. 2,3 Occasional cases of extramedullary plasmacytoma are reported in breast.