Osteosarcoma (OS) is a malignant bone tumor and it is very rare in head and neck region. If it arises in this area, most common localizations of this tumor are mandible and maxilla and ethmoid sinus is very rare site for this malignancy. A 43-year-old female admitted to our clinic with the complaint of pain in the right half of the face and headache persisting for 2 months. According to the clinical symptoms and imaging studies, sinonasal ossifying fibroma was suspected. For this reason, patient was operated endoscopically and histopathological examination revealed that osteoid matrix producing atypical osteoblastic cells so these findings identified osteoblastic variant OS. This article is about a very rare case of OS of ethmoid sinus. The differential diagnosis, general characteristics, and classification of this tumor, radiological features, and surgical techniques are also presented.