To our knowledge, this is the first reported case of a germinoma involving the midbrain without the demonstrable coexistence of any common midline tumors. A 27-year-old man was referred to our institution for evaluation and treatment of diplopia persisting for 5 years. Magnetic resonance imaging (MRI) showed the mass in the midbrain to be of iso intensity on T1-weighted images, and of high intensity on T2-weighted images with homogeneous enhancement. MRI-guided stereotactic biopsy was performed, and the histologic diagnosis was germinoma. Following biopsy, external beam radiotherapy of 50 Gy (whole brain 30 Gy: local 20 Gy) was performed. At the time of discharge, the patient's neurological symptoms had resolved. Follow-up MRI revealed disappearance of the tumor. These findings suggest the diagnostic value of magnetic resonance image-guided stereotactic biopsy in the differential diagnosis of adult brainstem lesions, which should now include germinoma.