Trans-mesenteric hernia is a form of internal hernia which is an extremely rare cause of intestinal obstruction in adults compared with the pediatric population. It often presents with complications such as acute intestinal obstruction and peritonitis requiring immediate surgical intervention. We report a case of a 21-year-old woman who presented to us with an acute abdomen and peritonitis that required an immediate exploratory laparotomy. She was found to have a small congenital mesenteric defect with strangulated segment of ileum. Gangrenous portion of the ileum was resected and the congenital defect was closed. The patient made an uneventful recovery. The occurrence of trans-mesenteric hernia in adults is rare and difficult to be diagnosed clinically; thus, the patient's clinical features may lead to early surgical intervention in order to reduce morbidity and mortality.