Sinonasal fibrosarcoma is an infrequently occurring malignant neoplasm. It usually presents with nasal obstruction and epistaxis, as do other sarcomas in this region. The final diagnosis is based on the histopathologic and immunohistochemical examination. We report a case involving a 47-year-old woman with a 2-year history of left nasal obstruction and proptosis, as well as diplopia for the 2 months preceding her visit. Computed tomography and magnetic resonance imaging showed a neoplasm occupying the left nasal cavity, ethmoid sinuses, and bilateral frontal sinuses. The neoplasm also was eroding the medial wall of the maxillary sinus, the lamina papyracea, the cribriform plate, and the anterior wall of the frontal sinus. Complete removal of the tumor was achieved both endoscopically and through a Lynch incision. Sinonasal fibrosarcoma was found on histopathologic examination.