Spontaneous isolated dissection of the superior mesenteric artery (SMA) is an uncommon phenomenon that rarely leads to a dissecting aneurysm. We herein report a case of a dissecting aneurysm of the SMA, which expanded rapidly and was successfully treated in emergency surgery. A 48-year-old male patient presented acute Type B aortic dissection in July 2012, since then he received annual to semi-annual follow-up computed tomography (CT) scans. Although the patient was asymptomatic, a CT in May 2017 revealed an isolated dissection of the SMA with a diameter of 14 mm. Approximately 17 months later, the SMA had dilated to 26 mm, forming a dissection aneurysm. One month later, the patient presented with epigastralgia, and the aneurysm was found to have further dilated to 32 mm. Emergency surgery was then undertaken for the rapid expansion of the aneurysm, and both the SMA and the second jejunal artery were reconstructed using the great saphenous veins. The surgery resulted in the patient recovering without any symptoms associated with intestinal ischemia. Since dissecting SMA aneurysms can expand rapidly, it is our view that careful observation should be mandatory for the patients with isolated SMA dissection.