Ectopic eruption of teeth rarely occurs in the mandibular condyle, orbit, nasal cavity, skin, and maxillary sinus. Ectopic eruption of teeth in the maxillary sinus can be accompanied by chronic and recurrent sinusitis, including headache, facial pain, and numbness. Fibro-osseous lesions in the paranasal sinuses are discovered incidentally on X-ray images and are often asymptomatic. Osteoma is the most common fibro-osseous lesion that develops in the paranasal and nasal sinuses. Herein, we report a rare case of osteoma (exostosis) containing supernumerary teeth within the maxillary sinus. A characteristic pedicled bone lesion with a clear border on computed tomography was the undefined orthopantomogram radiopacity in the left maxillary sinus, and the lesion contained teeth. The tumor was excised using the Caldwell–Luc procedure. Pathologically, the surgical specimen revealed an osteoma. The patient’s symptoms of chronic sinusitis disappeared, and the patient had no symptoms 2 years after the surgery. Because the patient had no history of midface trauma or surgery, the supernumerary teeth were speculated to have migrated during a reactive osteogenic process caused by chronic sinusitis.