A transmesocolic hernia is an uncommon type of internal hernia. We present a case of an internal hernia, through a congenital defect of the sigmoid mesocolon in a 3-year-old girl referred to our hospital with acute pain in the left abdominal region and vomiting. Because of the progressive worsening of her clinical status, an early laparotomy was performed and the intraoperative findings were consistent with a transmesosigmoid hernia. The hernia's sac contained gangrenous ileum loops, which were resected, and the defect was closed. Postoperative outcome was uneventful and the patient was discharged on the 10th postoperative day.