We present a 57-year-old male admitted to emergency department with acute loss of consciousness and diagnosed with third ventricular arachnoid cyst. Transcallosal cyst resection was performed following an emergency ventriculostomy. Postoperative imaging revealed gross-total cyst excision and a moderate decrease in hydrocephalus. However, the patient improved only after a subsequent ventriculoperitoneal shunting. This time, however, a subdural hematoma occurred under the craniotomy incision. In conclusion, surgical approach for the treatment of arachnoid cysts of the third ventricle should be selected carefully. Cyst excision via open craniotomy may require subsequent shunting and can cause serious complications such as subdural hematoma.