Superior mesenteric artery syndrome (SMAS) is an obstruction at the third portion of the duodenum by compression between the superior mesenteric artery and the aorta. In infancy, SMAS is extremely rare; and for its diagnosis, other duodenal obstructive diseases including congenital duodenal stenosis and intestinal malrotation must be ruled out. We present the case of a 7-month-old girl with frequent bilious vomiting after the resolution of acute gastroenteritis. Superior mesenteric artery syndrome was finally diagnosed at laparotomy, and duodenojejunostomy was performed. Vomiting disappeared postoperatively, and she gained weight. Although SMAS is an extremely rare syndrome in infants, it should be considered as a possible cause of incomplete duodenal obstruction.