Temporal crescent syndrome is a monocular visual field defect involving the temporal crescent of one eye caused by a retrochiasmal lesion. The most anterior portion of the striate cortex is the only area where the retrochiasmal lesion produces a monocular visual field defect. The authors present the case of a 9-year-old boy who presented with mild headache. MRI revealed a cyst with cerebrospinal fluid signal intensity, occupying the body and trigone of the right lateral ventricle. Conservative treatment with regular clinical and radiological follow-up was chosen because neurological examination findings were normal. Three years later, the patient experienced blurred vision with a temporal crescent defect in the left eye. Endoscopic cyst fenestration was performed, and the pathological findings indicated a glioependymal cyst. After surgery, the monocular temporal crescent disorder was resolved. MRI indicated shrinkage of the cyst and improvement in the narrowing of the anterior calcarine sulcus. These findings suggested that the temporal crescent syndrome was caused by a lateral ventricular glioependymal cyst. This is the first known report of temporal crescent syndrome caused by a lateral ventricular glioependymal cyst. In patients with monocular temporal crescent disorder without intraocular disease, a retrochiasmal lesion in the most anterior portion of the striate cortex should be considered.