Gastric outlet obstruction is commonly caused by gastric ulcers, pyloric stenosis, foreign bodies, and compression of the duodenum by pancreatic cancer and other intra-abdominal masses. There is no report on Riedel's lobe of the liver as a cause of this condition. This is the first report to describe Riedel's lobe of the liver as a cause of gastric outlet obstruction. An 81-year-old woman who complained of severe nausea and abdominal discomfort was admitted to our clinic. The physical examination revealed a mass in the right upper quadrant of the abdomen. An exploratory laparotomy showed an approximately 10x6-cm tongue-like mass arising from the left lobe of the liver, extending to the umbilicus and compressing the prepyloric area of the stomach. The nature and location of the mass were compatible with Riedel's lobe. A cholecystectomy was performed and the compressing mass was fixed to the abdominal wall with a simple suture. The patient did well postoperatively and was discharged from the hospital. We also reviewed the English-language literature to provide an update on this subject.