A 73-year-old woman was referred to our hospital complaining of bloody stool. She had undergone high anterior resection with the double stapling technique for a sigmoid colon cancer 2 years prior to this admission. Colonoscopy revealed a soft submucosal tumor, 4cm in size, on the anal side of the previous anastomosis in the rectum. EUS revealed a cystic lesion located in the third and fourth layers of the rectal wall. EUS-FNA was performed, and the content of the cystic lesion was transparent mucinous liquid. Histologically, the specimen revealed PAS and Alcian blue-positive mucinous material and a small number of inflammatory cells such as foamy macrophages. Therefore, this cystic lesion was diagnosed as a rectal implantation cyst.