A 24-year-old woman presented with a rare adrenocorticotropic hormone (ACTH)-positive pituitary adenoma in the cavernous sinus, manifesting as sudden onset of oculomotor nerve paresis. Neuroimaging revealed a tumor in the cavernous sinus bulging into the subdural space. There was no continuity between the tumor and the pituitary gland in the sella turcica. Gross total removal of the tumor was performed through the orbitozygomatic approach followed by stereotactic radiosurgery. The oculomotor nerve paresis was resolved. Histological examination revealed an adenoma positive for ACTH. Ectopic pituitary adenoma occurs mostly in the sphenoid sinus or the suprasellar region. This extremely rare case of ectopic macroadenoma in the cavernous sinus manifested as oculomotor nerve paresis without signs of Cushing's syndrome.