Pineal cysts of the third ventricle presenting with acute obstructive hydrocephalus due to internal cystic hemorrhage are a rare clinical entity. The authors report a case of a 61-year-old man taking antiplatelet medication who suffered from a hemorrhagic pineal cyst and was treated with endoscopic surgery. One month prior to treatment, the patient was diagnosed with a brainstem infarction and received clopidogrel in addition to aspirin. A small incidental pineal cyst was concurrently diagnosed using magnetic resonance (MR) imaging which was intended to be followed conservatively. The patient presented with a sudden onset of headache and diplopia. On admission, the neurological examination revealed clouding of consciousness and Parinaud syndrome. Computerized tomography (CT) scans demonstrated a hemorrhagic mass lesion in the posterior third ventricle. The patient underwent emergency external ventricular drainage with staged endoscopic biopsy and third ventriculostomy using a flexible videoscope. Histological examination revealed pineal tissue with necrotic change and no evidence of tumor cells. One year later MR imaging demonstrated no evidence of cystic lesion and a flow void between third ventricle and prepontine cistern. In patients with asymptomatic pineal cysts who are treated with antiplatelet therapy, it is important to be aware of the risk of pineal apoplexy. Endoscopic management can be effective for treatment of hemorrhagic pineal cyst with obstructive hydrocephalus.