Yolk sac tumor is the most common malignant neoplasm of germ cell origin and usually occurs in infant testes or ovaries. On rare occasions, the tumor arises from extragonadal sites, including the sacrococcygeal region, uterus, vagina, prostate, retroperitoneum, liver, mediastinum (commonly in the anterior), pineal gland, and third ventricle. Yolk sac tumors have an unfavorable prognosis, if not treated aggressively. We report the case of a 3-year-old boy with a primary posterior mediastinal yolk sac tumor who was managed initially with surgery, followed by chemotherapy and had a favorable prognosis. In the literature on yolk sac tumors presenting as a mediastinal mass, pediatric germ cell tumors have been reported very rarely in the posterior mediastinum.