Background Since the origin and growth pattern of third ventricle ectopic pituitary adenoma (ectPA) remain unclear, its optimal surgical approach is debatable. Clinical presentation We present a rare case of null cell pituitary adenoma arising from the pituitary infundibulum with long-term preoperative follow-up images. The tumor was resected gross-totally via an extended transsphenoidal approach. Conclusion To our best knowledge, this is the first case with long-term preoperative follow-up images, which can bridge the knowledge gap in operations of third ventricle ectPA as following: (1) Truly third ventricle ectPA can exist, (2) the third ventricle ectPA extended into the sella turcica along the pituitary stalk, (3) this ectPA can arise from the suprasellar peri-infundibular ectopic pituitary cells or the pars tuberalis of the adenohypophysis, and therefore adhere to the optic chiasm, (4) thus neurosurgeons should take great care in resection of ectPA arising from the infundibulum, and (5) it can be resected through an endoscopic extended transsphenoidal approach.