A 59-year-old male visited our department for examination of positive fecal occult blood test. The patient was scheduled for colonoscopy. However, he was admitted to our hospital urgently for rupture of left internal iliac artery aneurysm (IIAA) (Fig. 1). IIAA was managed by coil embolization and stent graft. Two weeks after the discharge, the patient visited our department because of intermittent hematochezia. Colonoscopy revealed a submucosal tumor-like protrusion with a fistulous tract in the sigmoid colon (Fig. 2a). Computed tomography demonstrated an iliac aneurysm with thrombus and air bubbles, fistulizing to the sigmoid colon (Fig. 2b). The patient was treated by an iliac artery aneurysm resection, stent graft removal, right common femoral artery–left common femoral artery bypass, and colostomy. Although iliac aneurysmo-colonic fistula is a rare condition, it can cause acute and massive lower gastrointestinal bleeding. In addition to arterial intervention such as endovascular therapy, abdominal or pelvic surgery, pelvic malignancy, and infection are also known causes of fistula formation. For the secondary aneurysmo-colonic fistula that occurs after endovascular therapy, as in our case, surgical treatment is often selected instead of repeat endovascular treatment. Our case suggests that aneurysmo-colonic fistula should be considered as one of the differential diagnoses among acute lower gastrointestinal bleeding, in patients with a history of recent endovascular therapy in large vessels in proximity to the colon.