Background: Malignant ovarian germ cell tumors are relatively rare, making up less than 2-3 % of all ovarian malignancies. However, although they represent only a small fraction, they frequently affect young women of reproductive age. Malignant germ cell tumors are subdivided into dysgerminoma and non-dygerminomatous tumors. The most common types of non-dysgerminomatous tumors are yolk sac tumors (YSTs). Case Report: The authors present a case report of a 32-year-old woman with a history of gradual abdominal distention and an increased serum level of alpha-fetoprotein (AFP). She was diagnosed with a pure YST after removing her left ovary. Then, she was subsequently treated with adjuvant chemotherapy by four cycles of bleomycin, etoposide, and cisplatin after fertility-sparing surgery. Conclusion: This case report shows that older patients could have this tumor with no associated tumor.