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A 14-year-old castrated male spitz dog was presented with intermittent coughing. Physical examination revealed bradycardia and a grade 3/6 left apical systolic murmur. Thoracic radiographs demonstrated severe bulging of the aortic arch with focal dorsal deviation of the trachea. Echocardiography revealed marked dilation involving the sinotubular junction and extending into the ascending aorta without discernible physiologic causes, with moderate aortic regurgitation and mild mitral regurgitation secondary to myxomatous degeneration. Electrocardiography showed a 2:1 second-degree atrioventricular block. Idiopathic thoracic aortic aneurysm was presumptively diagnosed. The dog was managed medically with pimobendan, theophylline, and famotidine. Clinical signs and electrocardiographic abnormalities improved, and the dog remained stable for 13 mo. Key clinical message: This case report demonstrates the diagnostic and therapeutic challenges and clinical relevance of idiopathic thoracic aortic aneurysm in dogs.