Arc of Riolan is a collateral channel that connects the proximal superior mesenteric artery (SMA) or its middle colic branch and the proximal inferior mesenteric artery or its left colic branch in case of stenosis of either of the arteries. A 65-year-old diabetic female presented with vague abdominal pain. Ultrasonography showed a large aneurysm within the abdomen in the left lumbar region. Computed tomography (CT) angiography done showed severe diffuse atherosclerotic calcification of the abdominal aorta with complete occlusion of the celiac trunk and mild stenosis of SMA origin. The arc of Riolan was seen between the middle colic artery and the ascending branch of the left colic artery, with a large saccular aneurysm in its mid section. No evidence of rupture or hematoma was visible. Another saccular aneurysm was also seen involving the gastro-duodenal and the pancreatico-duodenal collateral arcade. As far as we know, this is the first case of arc of Riolan artery aneurysm to be reported in English literature.