Mobile plaque is associated with increased risk of ischemic stroke, but definitions have remained unclear. We have previously reported that carotid ultrasonography can detect the mobile component of the carotid plaque surface, which rises and falls in a manner inconsistent with arterial pulsatile wall motion (Jellyfish sign). However, clinical and pathological features of Jellyfish sign remain unclear. The subjects comprised of 165 patients with carotid plaque and degree of area stenosis ≥50% on ultrasonography. Using magnetic resonance imaging, we quantified intraplaque hemorrhage (IPH) and defined ischemic stroke in each patient. Fifteen surgical specimens were obtained by carotid endarterectomy, and pathological features (area of fibrous cap and intraplaque atheromatous lesion) were compared with ultrasonographic plaque surface movement rate. Carotid plaques with IPH were seen in 78 cases, with Jellyfish sign in 31 cases. Jellyfish sign was not detected in patients without IPH. In these 15 patients, the fibrous cap covered the atheromatous lesion, and cap thickness correlated negatively with Jellyfish-positive plaque surface movement rate. Kaplan-Meier and Cox multiple regression analysis demonstrated that the most important predictor of ischemic stroke during follow-up is Jellyfish sign, not IPH. Stroke events in patients with Jellyfish sign repeated within a short interval after diagnosis. Jellyfish sign on ultrasonography is a sign of high-risk plaque vulnerability, suggesting rupture of the fibrous cap associated with the release of thrombogenic factors into the arterial lumen, and resulting in repeated ischemic stroke during a short interval after diagnosis.