Extramedullary plasmacytomas are rare malignancies. Most cases (80%) are seen in the head and neck region, where they represent 1% of all head and neck malignancies. We report a case of an extramedullary plasmacytoma of the tongue that was treated successfully with three-dimensional conformal radiotherapy. The patient was a 50-year-old woman who was admitted to our hospital with bilateral pain and ulceration on the sides of her tongue. Findings on magnetic resonance imaging and positron-emission tomography did not demonstrate any mass within the tongue or any lymphatic or distant metastasis. A tissue biopsy identified a plasma cell neoplasm. The patient was treated with a total dose of 50 Gy delivered in 2-Gy daily fractions. After 54 months of post-treatment follow-up, she exhibited no sign of systemic myeloma or local recurrence. This case is presented not only for the rarity of the tumor type, but also for its unusual location.