Background Sellar/suprasellar cystic lesions can cause compression of the optic pathways and result in a decline in vision. There have been reports of optic pathways being fenestrated by intracranial aneurysms. This is the first report of a sellar/suprasellar arachnoid cyst causing fenestration of an optic nerve. Case Description Our patient presented with an 8-month history of worsening vision. Imaging revealed a sellar/suprasellar cystic lesion with compression of the optic pathways. He was treated surgically via an endoscopic transsphenoidal approach. The left optic nerve was found to have multiple round defects during surgery. Pathology was consistent with an arachnoid cyst. The patient has had progressive improvement in vision up to 6 months after surgery. Conclusions Sellar/suprasellar arachnoid cysts likely form when a communication exists between the suprasellar arachnoid space and the sella turcica. We believe that our patient's cyst grew slowly enough to allow normal functioning of the optic nerve as it was being penetrated. Though visual symptoms may be gradual and not present until after the optic nerve has been penetrated, these symptoms and signs may still improve with removal of the cyst. Prognosis for visual improvement is difficult to predict with cases of compressive sellar/suprasellar cystic masses. Improvement in the vision examination after surgical resection is possible even when the optic nerve has been penetrated.