Multiple myeloma, solitary plasmacytoma, and extramedullary plasmacytoma constitute a continuous disease spectrum of plasma cell neoplasms. In the larynx, although extramedullary plasmacytoma in the supraglottic region has been sometimes reported, plasma cell neoplasm with involvement of the thyroid cartilage is extremely rare. We report a case of multiple myeloma involving the thyroid cartilage. A 72-year-old male patient presented with a pathological fracture of the cervical vertebrae. CT scan revealed low-density areas within the thyroid cartilage, but the laryngeal mucosa and submucosal soft tissue were intact. Multiple myeloma was diagnosed, since the tumors in the thyroid cartilage and cervical vertebrae revealed plasmacytoma. Because no other lesion was found, irradiation of the larynx and cervical vertebrae was performed. Neither aggressive change of these lesions nor new lesions have been found over 3-year follow-up following the initial treatment without adjuvant therapy.