Alimentary tract duplications are uncommon congenital abnormalities that usually have an anatomical connection with some part of the gastrointestinal tract and have a common blood supply with the adjacent segment of intestine. A completely isolated duplication cyst (CIDC) is a very rare type of gastrointestinal duplication that does not communicate with the normal bowel segment and possesses its own exclusive blood supply. Only 5 CIDC cases in adults have been reported in the English medical literature. Additionally, only 1 case of mucinous cystadenoma from an infected CIDC of the ileum has been reported. This report describes a 52-year-old male patient with a peritoneal CIDC, which upon curative excision was found to have given rise to an adenocarcinoma. The latter was lined internally with malignant glandular cells and contained a smooth muscular outer layer as determined by microscopic examination of the tissue. We believe that this is the first reported case of an adenocarcinoma originating from a CIDC in an adult.