Adenomyoma of the gastrointestinal tract is a rare benign tumor-like lesion. The small intestine is the second most frequent location, usually in the periampullary area, but the lesion also occurs in the jejunum and ileum. While adenomyoma of the Vaterian system is primarily diagnosed in adults, more than half of reported cases of jejunal and ileal adenomyoma have been diagnosed in pediatric patients. Adenomyoma of the periampullary area usually presents with biliary obstruction or abdominal pain, whereas jejunal and ileal adenomyoma usually presents with intussusception or is incidentally discovered during surgery or autopsy. Since endoscopic and radiological examination yields uncharacteristic findings, histopathological evaluation is important in adenomyoma diagnosis. Pathologically, adenomyoma consists of glandular structures of various sizes and interlacing smooth muscle bundles that surround the glandular elements. The pathogenesis of adenomyoma is generally considered to be either a form of hamartoma or a pancreatic heterotopia. Although limited resection is considered the most effective treatment, pancreaticoduodenectomy is often performed when the lesion occurs in the periampullary area due to preoperative misdiagnosis as a carcinoma. It is, therefore, important that clinicians and pathologists maintain current knowledge of the disease to avoid inaccurate diagnosis, which could lead to unnecessary surgery.