Plasmacytomas of the thyroid are uncommon. A 19-year-old female presented with a palpable, 3.5-cm-diameter mass in the right thyroid lobe. Fine-needle aspiration cytology (FNAC) showed that lymphoma cells were likely. The patient underwent a right lobectomy. Final haematoxylin and eosin (HE) staining of the tumour confirmed a diagnosis of thyroid plasmacytoma. Immunohistochemical staining showed that plasma cells were stained strongly with IgA antibody. To date, this is the youngest patient with thyroid plasmacytoma in the literature. Diagnosis of thyroid plasmacytoma by fine-needle aspiration cytology is typically difficult, as it was for this patient. Currently, no treatment standard exists for thyroid plasmacytoma.