Segmental arterial mediolysis (SAM) is an uncommon, nonarteriosclerotic vascular disease. SAM is characterized by lysis of arterial media and can lead to aneurysm formation. The renal arteries are the third most common arteries associated with SAM. We report the case of a 32-year-old man with left renal artery aneurysm associated with SAM. We successfully performed left renal autotransplantation using microscopic vascular reconstruction. SAM is characterized by vascular fragility; therefore, microscopic surgery is favorable for treating aneurysms associated with SAM.