Background: Yolk sac tumor (YST) is the second most common malignant ovarian germ cell tumor, while a YST arising in the omentum is an exceedingly rare malignancy. Case: A 44-year-old woman was admitted with a history of abdominal distension of a month’s duration. The alpha-fetoprotein (AFP) serum level was elevated to 27,612 ng/ml, and CT scanning reported an obviously thickened omentum. Explorative findings revealed a large omental mass with a small implantation on the surface of the left ovary. Histological evaluation of the specimen after surgery exhibited typical patterns of YST, and the specimen was stained for AFP, cytokeratin, and placental alkaline phosphatase. The patient was treated with total abdominal hysterectomy with bilateral salpingooophorectomy and infracolic omentectomy followed by four cycles of bleomycin, etoposide, and cisplatin combination chemotherapy. She has remained free of the disease for seven months after completion of therapy. The subject of YST arising in the omentum is reviewed. Conclusion: This is the fourth case of primary omental YST in females. The case of omental YST must be seriously considered once the tumor shows omentum thickening with elevated AFP serum levels.