Introduction: The correlation between the composition of thrombi retrieved by mechanical thrombectomy (MT) and stroke etiology is inconclusive. We describe a case with atherosclerotic components in thrombi retrieved by MT for acute internal carotid artery (ICA) occlusion. Case Presentation: A 69-year-old man with acute onset of global aphasia and right hemiplegia was transferred to our institute. His baseline National Institutes of Health Stroke Scale score was 24. Magnetic resonance imaging demonstrated acute ischemic stroke in the left parietal lobe. Magnetic resonance angiography revealed occlusion of the left ICA. MT was attempted for acute left ICA occlusion. The initial angiography showed occlusion of the proximal ICA, while intraprocedural angiography revealed a large thrombus that extended from the cervical ICA to the intracranial ICA. Successful reperfusion was achieved by five passes using stent retrievers and an aspiration catheter. A large volume of red thrombus was retrieved by each pass. The final angiogram showed successful reperfusion with modified Thrombolysis in Cerebral Ischemia grade 2b and severe stenosis in the proximal ICA. Neck magnetic resonance imaging showed severe left ICA stenosis with a vulnerable plaque. Hence, his stroke etiology was determined as large artery atherosclerosis. Histopathological examination of the retrieved thrombi revealed atheromatous components, including cholesterol clefts, foam cells, and a necrotic core. Conclusions: Atherosclerotic components in retrieved thrombi might provide useful clues for diagnosing stroke pathogenesis. Further studies are warranted to clarify the utility of assessing atheromatous components in retrieved thrombi in diagnosing stroke etiology.