1208?–1265, leader of the baronial revolt against Henry III of England.
He was born in France, the son of Simon de Montfort, leader of the Albigensian Crusade. After his father's death, he received the claim to the earldom of Leicester, inherited from his grandmother. He went to England in 1229, and two years later his earldom was recognized by Henry III. He became one of the king's advisers and in 1238 married Eleanor, Henry's sister. In 1240, Simon distinguished himself on crusade in Palestine under Richard, earl of Cornwall.
Returning to France in 1242, he joined Henry III in the Gascon campaigns of 1242–43. Simon was preparing to go on a new crusade when in 1248 Henry sent him to Gascony with unlimited powers to bring order out of the anarchy of petty feudal wars and rebellions against English authority. Simon was skillful and ruthless in using military force to crush the turbulent Gascon barons and achieved a somewhat unstable order. But loud Gascon protests provoked Henry in 1252 to call Simon to an inquiry in England. After a bitter quarrel with the king was temporarily ended, Simon returned to Gascony, only to be interrupted a second time by a royal order to desist in the middle of his campaign so that young Prince Edward (later Edward I) might take Gascony in charge.
By 1258 Simon was an active member of the baronial opposition that forced the king to turn over the power of government to a committee of 15 (of whom Simon was one), which ruled under the Provisions of Oxford, supplemented by the Provisions of Westminster of 1259. Divisions soon appeared in the baronial party, and in 1261, when a majority of the barons consented to an unfavorable compromise with the king, Simon left England. There was, however, renewed discontent in England following Henry's annulment (1262) of the provisions, and in 1263 Simon returned to assume leadership in the Barons' War.
Simon won a great victory at Lewes in 1264 and became master of England, which he intended to place under a form of government similar to that prescribed in the Provisions of Oxford. However, he could achieve no legal settlement with the king and so ruled as virtual military dictator. His famous Parliament of 1265, to which he summoned not only knights from each shire but also, for the first time, representatives from boroughs, was an attempt to rally national support, but at the same time he was alienating many of his baronial supporters. In 1265 his most powerful ally, Gilbert de Clare, 8th earl of Gloucester, deserted and with Prince Edward joined the nobles of the Welsh Marches to start the wars again. Simon de Montfort was defeated and killed at Evesham.