An 83-year-old woman with hypertension and hypothyroidism presented to the emergency department with a 1-day history of swelling on the right side of her tongue. On examination, her heart rate was 120 beats per minute, and her blood pressure was 145/90 mm Hg. Her tongue was smooth, swollen, and erythematous, with well-demarcated dark discoloration on the right side. No other abnormalities were found in the oral cavity, pharynx, or larynx. The results of sensory examination of the tongue were normal. An electrocardiogram showed atrial fibrillation. Right lingual-artery thromboembolism in the presence of newly diagnosed atrial fibrillation was suspected. Computed tomographic angiography was performed, and the result confirmed the diagnosis. Dexamethasone and anticoagulation therapy were initiated. The tongue swelling and discoloration resolved completely within 8 days. On follow-up at 6 months, no motor or sensory sequelae were noted.