Extramedullary plasmacytoma is a type of plasma cell dyscrasia that can present as solitary tumor or secondary to multiple myeloma. We experienced a case of intramuscular plasmacytoma in the left thigh muscles of a patient secondary to multiple myeloma. A 73-year-old male with relapsed multiple myeloma and bilateral hip arthroplasty complained of lxeft lower limb weakness and hip pain 3 months after relapse. He underwent contrast-enhanced magnetic resonance imaging of lumbar spine and hip which was inconclusive. Subsequently, patient had multiple admissions for progressive lower limb weakness. His clinical course was complicated by a biopsy-proven plasmacytoma of the neck. He received localized radiation therapy to the neck in addition to a change in multiple myeloma chemotherapy regimen, resulting in resolution of the neck mass but his left lower extremity weakness continued to worsen. Repeat magnetic resonance imaging of hip and spine revealed an intramuscular mass in left thigh which was consistent with the diagnosis of extramedullary plasmacytoma on biopsy. Localized radiation to the thigh accompanied with a change in chemotherapy improved his symptoms and a significant reduction in size of plasmacytoma was observed. When an unexplained lower limb weakness is encountered with a history of multiple myeloma, secondary intramuscular plasmacytoma should be considered.