We report a case of adenocarcinoma developing in remnant rectal mucosa below a hand-sewn ileal pouch-anal anastomosis (IPAA) after restorative proctocolectomy for ulcerative colitis (UC). To our knowledge, this is the first such case to be reported from Japan. A 60-year-old man with a 13-year history of UC underwent proctocolectomy with a hand-sewn IPAA and mucosectomy for anal stenosis and serious tenesmic symptoms. About 7 years later, a follow-up endoscopy showed a flat elevated malignant lesion, 2 cm in diameter, below the ileoanal anastomosis. He was treated by abdominoperineal resection of the pouch and anus with total mesorectal excision. Histopathological examination of the resected specimen confirmed the presence of a well-differentiated adenocarcinoma but there were no metastatic lymph nodes. He recovered uneventfully and remains well without evidence of recurrent disease 2 years and 3 months after his last operation.