A 69-year-old woman, a long-term survivor of subtotal stomach-preserving pancreatoduodenectomy with the splenic vein resection for pancreatic cancer, visited our hospital with a chief complaint of bloody stools. Previously, she was diagnosed with varices in the ascending colon due to left-sided portal hypertension after pancreatoduodenectomy by computed tomography and colonoscopy. After emergency hospitalization, she went into shock, and blood tests showed acute progression of severe anemia. Computed tomography showed a mosaic-like fluid accumulation from the ascending colon to the rectum. She was diagnosed with ruptured varices in the ascending colon. Emergency colonoscopy was performed, and treatment with endoscopic injection sclerotherapy using N-butyl-2-cyanoacrylate was successful. Ectopic varices occur at any location other than the esophagus and stomach, and colonic varices are rare among them. They are mostly caused by portal hypertension due to liver cirrhosis. However, with the trend of improving the prognosis for patients with pancreatic cancer, we should occasionally pay attention to the development of ectopic varices including colonic varices in patients who have undergone pancreatoduodenectomy with superior mesenteric and splenic veins resection. Treatment methods for colonic varices varied from case to case, including conservative therapy, interventional radiology, and endoscopic procedure. In this case, endoscopic injection sclerotherapy was successfully performed without any complications. To the best of our knowledge, this is the first study to report successful treatment with endoscopic injection sclerotherapy for varices in the ascending colon caused by left-sided portal hypertension after pancreatoduodenectomy. Colonic varices should be considered in patients with obscure gastrointestinal bleeding after pancreatoduodenectomy.