Extramedullary plasmacytoma and tooth eruption into the nasal cavity are both rare events. We report a case of plasmacytoma associated with an ectopic tooth. To the best of our knowledge, such a case has not been previously reported in the literature. A 63-year-old woman presented for evaluation of an 8-month history of a bloody nasal discharge from the posterior naris. Nasal endoscopy detected a slight eminence approximately 0.5 cm in diameter in the right nasal floor. Computed tomography demonstrated a tooth-like, high-attenuation shadow. A biopsy identified chronic inflammation of the mucosa and tissue. A diagnosis of an ectopic tooth in the right nasal cavity was initially considered. The mass and the tooth-like neoplasm were removed via nasal endoscopy. Immunohistochemistry of the excised mucosa showed strong positivity for kappa light chains, positivity for leukocyte common antigen and CD138, and negativity for lambda light chains, epithelial membrane antigen, CD1, and HMB-45. The final diagnosis was an extramedullary plasmacytoma in the right nasal cavity associated with an ectopic tooth. No bone metastasis was observed. Definitive radiotherapy was performed after the operation. During 40 months of follow-up, the patient exhibited no evidence of local recurrence or metastasis. The diagnosis in this case was made difficult by the nonspecific clinical manifestations, the presence of the ectopic tooth, and incorrect interpretation of preoperative histopathology. Physicians should maintain a clinical suspicion for the possibility that an ectopic tooth might be associated with a tumor.