Giant pituitary adenomas carry significant surgical risks when treated with transsphenoidal approaches or the transcranial approach alone. Combined transsphenoidal and transcranial approaches have been reported; however, removing adenomas extending into the third ventricle may still be challenging. We report a case of giant pituitary adenoma expanding into the third ventricle, which was removed using a combined transventricular preforniceal approach and an endoscopic endonasal transsphenoidal surgery (ETSS). A 41-year-old man with headache, nausea, and a 1-week history of a visual field defect was transferred to our hospital. He had a disturbed left visual acuity, right homonymous hemianopia, and choked disc in both eyes. Preoperative magnetic resonance imaging revealed a giant pituitary adenoma with a maximum diameter of 55 mm extending from the intrasellar to the suprasellar region, thus occupying the entire third ventricle and causing hydrocephalus. The space between the anterior commissure and the fornix was expanded. The foramen of Monro was shifted backward due to compression by the tumor. He underwent maximum surgical resection using a combined transventricular preforniceal approach and ETSS. Considering technical demands and reliability, the intra- to suprasellar parts were removed by ETSS while the intraventricular part was removed through the preforniceal approach. The residual tumor in the right cavernous sinus and behind the anterior communicating artery was treated with stereotactic radiotherapy. One year after the operation, the patient leads an independent life. The combined technique of the preforniceal approach and ETSS provided a direct view of the entire third ventricle and hemostasis in the present case.