A 2-year-old boy presented with gait disturbance and limb ataxia. T1-weighted magnetic resonance (MR) imaging with gadolinium-diethylenetriaminepenta-acetic acid administration showed a heterogeneously enhanced mass lesion with a cystic component in the cerebellar vermis. The minimum apparent diffusion coefficient value of the lesion was 1.96 x 10(-3) mm2/sec, and 1H-MR spectroscopy showed elevated choline and lipid peaks, and decreased N-acetyl aspartate peak. The tumor was totally resected, and the histological diagnosis was yolk sac tumor. Consistent with this diagnosis, a-fetoprotein levels in the serum and cerebrospinal fluid were 7094 ng/m/ and 22.3 ng/m/, respectively. 18F-fluorodeoxyglucose-positron emission tomography, and thoracic, abdominal, and pelvic computed tomography showed no abnormal lesions, excluding the possibility of metastatic yolk sac tumor from an extracranial lesion. The patient received chemotherapy consisting of ifosfamide, cisplatin, and etoposide, and had not relapsed at 6 months after resection. Germ cell tumors rarely develop in the posterior fossa. This case suggests that yolk sac tumor could develop in the cerebellar vermis.