Cysts occupying the third ventricle are rare lesions and may appear as an unusual cause of obstructive hydrocephalus. Various types of lesions occur in this location, and they generally have an arachnoidal, endodermal, or neuroepithelial origin. The authors present a case of acute hydrocephalus following minor trauma in a child due to cerebrospinal fluid outflow obstruction by a third ventricular cyst. Definitive diagnosis of this cystic lesion was possible only with contrast ventriculography and not routine computed tomography or magnetic resonance imaging. The investigation, treatment, and pathological findings are discussed.