Idiopathic granulomatous hypophysitis (IGH) is a rare inflammatory disease of the pituitary that commonly presents with enlargement of the pituitary gland. Clinically and radiologically, IGH is a rare sellar entity easily to be misdiagnosed as a pituitary adenoma. Through such a case, we aim to present this rarity and emphasize the importance to correctly diagnose confusing pituitary lesions comprehensively by clinical presentations, radiological signs, and biopsy. We present an uncommon case of IGH in a 19-year-old man. The patient was admitted to the hospital with severe headache, vomiting, and vision's sharp decline. Magnetic resonance imaging showed a sellar lesion with obvious cystic change and ring enhancement. The disease course including diagnosis and treatment was presented and analyzed in detail. The pertinent literature is reviewed regarding this uncommon entity. The patient underwent surgical exploration and partial resection via the transsphenoidal approach. The pathologic findings suggested IGH giving no significant evidences of systemic granulomatous disease and venereal disease. Large dose methylprednisolone was then used. The pituitary function recovered, but there was no apparent improvement of his vision. IGH is a rarely occurred inflammatory disease of unknown etiology. It is difficult to diagnose preoperatively and is often misdiagnosed. Although rare, IGH should be kept in mind in terms of differential diagnosis of sellar region lesions.