The Barons' Crusade (1239–1241), also called the Crusade of 1239, was a crusade to the Holy Land that, in territorial terms, was the most successful crusade since the First Crusade. Called by pope Gregory IX, the Barons' Crusade broadly embodied the highest point of papal endeavor "to make crusading a universal Christian undertaking." Gregory IX called for a crusade in France, England, and Hungary with different degrees of success. Although the crusaders did not achieve any glorious military victories, they used diplomacy to successfully play the two warring factions of the Ayyubid dynasty (as-Salih Ismail in Damascus and as-Salih Ayyub in Egypt) against one another for even more concessions than Frederick II had gained during the more well-known Sixth Crusade. For a few years, the Barons' Crusade returned the Kingdom of Jerusalem to its largest size since 1187.

This crusade to the Holy Land is sometimes discussed as two separate crusades: that of King Theobald I of Navarre, which began in 1239; and, the separate host of crusaders under the leadership of Richard of Cornwall, which arrived after Theobald departed in 1240. Additionally, the Barons' Crusade is often described in tandem with Baldwin of Courtenay's concurrent trip to Constantinople and capture of Tzurulum with a separate, smaller force of crusaders. This is because Gregory IX briefly attempted to redirect the target his new crusade from liberating the Holy Land from Muslims to protecting the Latin Empire of Constantinople from "schismatic" (i.e., Orthodox) Christians attempting to retake the city.

Despite relatively plentiful primary sources, scholarship until recently has been limited, due at least in part to the lack of major military engagements. Although Gregory IX went further than any other pope to create an ideal of Christian unity in the process of organizing the crusade, in practice the crusade's divided leadership did not reveal a unified Christian action or identity in response to taking the cross.