<p>Objective: We report a case of acute internal carotid artery occlusion in a patient with adult-onset moyamoya disease who underwent mechanical thrombectomy and had a good outcome.</p><p>Case Presentation: A 73-year-old woman was diagnosed with moyamoya disease by asymptomatic right middle cerebral artery occlusion at 59 years of age. The patient was transported for stroke symptoms. Magnetic resonance imaging (MRI) demonstrated left terminal internal carotid artery occlusion and low-intensity signal on T2^*-weighted imaging at the occlusion site. Alteplase was administered and endovascular treatment was subsequently performed. A small-diameter microcatheter was guided to the distal end of the occlusion and angiography after deployment of a stent retriever revealed irregular stenosis. Severe stenosis remained after thrombectomy, and balloon angioplasty was added. The treatment resulted in recanalization and good outcome.</p><p>Conclusion: Adults with moyamoya disease may have accompanying atherosclerotic intracranial artery occlusion. Angiography after deployment of a stent retriever was useful for clarifying the etiology of occlusion. It is important to determine the etiology of occlusion based on the medical history or imaging findings and to select an appropriate treatment.</p>