We report a case of ileal internal hernia through the foramen of Winslow into the lesser sac. Preoperative computed tomography (CT) demonstrated that the herniated ileum, which showed a closed-loop obstruction, was located behind the portal vein, and the vein was subsequently compressed and narrowed by the herniated ileum. We found that similar cases in the literature of Winslow's foramen hernias that caused portal vein compression; however, portal vein narrowing has not been described as a characteristic CT finding. The narrowed portal vein sign could be useful in diagnosing the hernia through the foramen of Winslow.