orig. Ugo di Segni
born before 1170
died Aug. 22, 1241, Rome
Pope (1227–41) who founded the papal Inquisition .
In 1227 he excommunicated Frederick II when the emperor delayed in keeping his pledge to lead a Crusade. Gregory ordered an attack on the kingdom of Sicily in the emperor's absence, but his forces were defeated. In 1234 he published the Decretals, a code of canon law that remained fundamental to Catholicism until World War I. Attacking heresy in southern France and northern Italy, he strengthened the Inquisition. Frederick's invasion of Sardinia, a papal fief, led Gregory to renew his excommunication (1239); he sought support in northern Italy but died before the struggle was resolved.