This is the first report of a carotid aneurysm that developed from a cavernous carotid artery contiguous with a prolactinoma during medical treatment of the prolactinoma, which gradually grew larger while the tumor regressed. A 78-year-old woman presented with headache and neurological symptoms indicating the involvement of cranial nerves in the cavernous sinus. Gadolinium-enhanced T1-weighted magnetic resonance imaging on admission revealed an abnormal right cavernous sinus, with an approximately 17 mm mass extending into the right cavernous portion of the internal carotid artery, and was contiguous with the intracavernous carotid artery. She was diagnosed with pituitary apoplexy due to a prolactinoma and started cabergoline treatment. After medical treatment, a carotid aneurysm emerged. The aneurysm continued to grow and reached a maximum diameter of 10.4 mm at 81 months after the initiation of treatment. The patient underwent endovascular coil embolization, following which the aneurysm regressed. Association between a prolactinoma and the development of a contiguous aneurysm remains undetermined. However, this is an odd phenomenon, and to the best of our knowledge, this is the first reported case of the development of an aneurysm that was associated with a pituitary tumor.