Introduction and importance: Primary internal hernias are rare in adults. Internal hernias present clinically as a sign of small intestinal obstruction. If left untreated, internal hernias might lead to high morbidity and mortality due to strangulation. Internal hernias are usually diagnosed intraoperatively. Here, we present an internal hernia diagnosed with an abdominal computed tomography (CT) scan. The importance of preoperative diagnosis for internal hernias leads to early surgery and keeps the patient from suffering intestinal strangulation. Case presentation: We report here the case of a 67-year-old male who presented with acute intestinal obstruction and underwent an abdominal CT scan. The patient was diagnosed with an internal hernia from the imaging of the abdominal CT scan and scheduled for an exploratory laparotomy. An internal hernia was found in the mesocolon of the sigmoid colon; one loop of jejunum was trapped in the hernia defect. After reduction, the hernial defect was closed; no resections were done, and the patient was discharged after 5 days without complication. Clinical discussion: Our finding represents a transmesosigmoid hernia, a rare variant of sigmoid mesocolon hernias. The clinical sign and the judgment of the surgeon for the diagnosis of an internal hernia became important factors related to the patient’s outcome. Conclusion: The proper adjunct imaging, correct diagnosis, and timing of surgery for internal hernias save the patient from morbidity or intestinal death.