A 57-year-old man with multiple myeloma had a rapidly enlarging mass in the right thyroid lobe for 6 months. Fine needle aspiration biopsy was performed, and cytologic examination revealed atypical plasma cells. One week later, the patient was referred for F-18 FDG imaging for further investigation and hyperactive foci on the right thyroid lobe were detected. We are reporting a case with extramedullary plasmacytoma of the thyroid detected on PET/CT scan.