OBJECTIVE: Osteoblastoma is a benign bone tumor that generally occurs in the vertebral column or in the long bones of the extremities. Involvement of the nasal and paranasal sinuses is rare. CASE REPORT: We describe a case of a benign osteoblastoma that developed in the ethmoid sinus of an 11-year-old girl. Her chief symptoms were progressive left nasal obstruction and facial asymmetry. She also complained of eventual epistaxis and facial pain, which did not correlate with the time of the day. Computed tomography (CT) showed an expansive mass having the same density as bone with an irregular border in the left ethmoid cells, extending to the frontoethmoid cells, the left nasal cavity, the left maxillary sinus, and the medial wall of the left orbit, causing deviation of the ocular globe. The maximum diameter of the lesion was 3.1 cm. Complete removal was accomplished under endoscopic view and using an open surgical excision approach. Histologic inspection showed osteoid tissue with richly vascular, fibrous stroma. The diagnosis was based on clinical presentation, histopathologic findings, and the size of the tumor. No recurrence was evident at the 3-month follow-up visit. CONCLUSION: Osteoblastoma of the paranasal sinuses is rare. Although it is a benign tumor, the lesion grows in a locally aggressive manner. Differential diagnoses consist of other kinds of fibro-osseous lesions, like osteoid osteoma. The osteoblastoma diagnosis was based on the clinical features, microscopic, and macroscopic findings. Complete resection of the lesion is the preferred treatment.