A 16-year-old woman identified with colonic distention using chest X-rays visited our hospital. Although abdominal computed tomography (CT), colonoscopy, and barium enema study indicated suspected duplication of the sigmoid colon, the exact portion of communication between the normal colon and the duplicated colon could not be determined. The patient was released, but followed up due to the lack of symptoms. After 7 months, she was urgently re-hospitalized due to the complaint of abdominal pain. Her abdominal CT revealed the wall thickness and distention of the duplication as well as voluminous stool containing barium. After the improvement of her symptoms and on the basis of the inflammatory findings, laparoscopic surgery was performed on the patient. Finally, the lesion was diagnosed as tubular- and continuous-type colonic duplication. Duplication of the colon is a relatively rare occurrence in adulthood. Herein, we report a case of duplication of the sigmoid colon diagnosed prior to surgery in an adult.