Extramedullary plasmacytomas most commonly occur in the nasal cavity, nasopharynx, paranasal sinuses, and larynx. Thyroid involvement is rare, as fewer than 75 cases have been previously reported in the literature. We report a new case of disseminated plasmacytoma of the thyroid, which occurred in a 68-year-old woman. The diagnosis was made with an incisional biopsy. The patient initially experienced a complete response to radiotherapy (46.8 Gy in 26 fractions), but she developed multiple metastases at distant sites, including the breast, abdominal wall, and buttock despite various chemotherapy regimens. Ultimately, she was placed on palliative radiotherapy. We also briefly review the diagnostic and treatment options for patients with extramedullary plasmacytoma.