A 61-year-old woman underwent laparotomy because of a right ovarian cyst 25 cm in diameter and clinically benign. Unexpectedly, a workup showed primary squamous cell carcinoma of the ovary. The patient received adjuvant radiation therapy. Follow-ups have not revealed any signs of relapse. The histogenesis of this rare tumor is uncertain, but the diagnosis is important because the prognosis and the response to adjuvant therapy of primary ovarian squamous cell carcinoma is probably worse than for other ovarian epithelial carcinomas.