Trousseau's syndrome is characterized by a cerebral or systemic thromboembolism caused by coagulation abnormalities in malignancy. We herein report a case of multiple ischemic strokes as the initial manifestation of anaplastic thyroid carcinoma (ATC). An 86-year-old man was admitted to our hospital due to a sudden-onset weakness of the left limbs. Brain magnetic resonance imaging revealed multiple ischemic lesions in the right middle cerebral artery territory and a mass in the left frontal lobe. Computed tomography revealed a thyroid mass and multiple lung tumors. A diagnosis of ATC was confirmed by a thyroid biopsy. Our case indicates that ATC should be considered as a cause of Trousseau's syndrome.