Simon de Montfort, 6th Earl of Leicester (1208 - August 4, 1265) was the principal leader of the baronial opposition to king Henry III of England.

He was the younger son of Simon de Montfort, a French nobleman, and Amicia de Beaumont. His mother was senior co-heiress to the Earldom of Leicester and a large estate in England, but king John of England would never allow anyone who already held property in France to take ownership of such an estate in England.

When the elder Simon died in 1218, he was succeeded by the elder son Amaury. In 1229 the two brothers (Amaury and Simon) came to an arrangement whereby Simon gave up his rights in France and Amaury in turn gave up his rights in England. Thus freed from any any allegiance to the king of France, Simon successfully petitioned for the English inheritance, which was granted the next year, although he did not take full possession for several more years, and was not yet formally recognized as earl.

Meanwhile in 1238 he secretly married Eleanor, sister of King Henry III of England. Eleanor had previously been married to William Marshal, Earl of Pembroke, and had sworn a vow of chastity on his death, which she broke by marrying Simon. Once reconciled to the match, the king recognized Simon as Earl of Leicester, but they frequently disagreed.

Like his father, Simon was a hardened and ruthless soldier, as well as a capable administrator. The main cause of his dispute with the king was the latter's determination to ignore the swelling discontent within the country, caused by a combination of factors which included famine. In 1258, at Oxford, in the moment he is most famous for Simon was instrumental in calling a parliament which is regarded as the forerunner of the modern institution. The king's son, the future King Edward I of England was at first sympathetic to Simon's cause, but later they became enemies, and the Provisions of Oxford, which the king had sworn to uphold, were broken at the behest of the Pope in 1261.

Civil war broke out, and Simon de Montfort's army met and defeated the royal forces at the Battle of Lewes in 1264. Prince Edward was taken prisoner, and the subsequent treaty set up a model parliament to agree a constitution formulated by Simon. However, his many enemies turned his triumph into disaster, and he was defeated and killed by Edward's forces during the Barons' War at the Battle of Evesham in 1265, his body being mutilated, eviscerated and the remains scattered. His family were forced into exile in Simon's native France. His daughter, Eleanor, later married Llywelyn the Last as Simon had planned.

After his death Simon's lands and titles were forfeited to the crown. A few months later they were re-granted to Edmund Crouchback, the king's youngest son.

De Montfort has given his name to various English institutions, such as De Montfort University and De Montfort Hall, both in Leicester.

There is a memorial to Simon de Montfort in the park in Evesham in a place believed to be near where the High Altar of Evesham Abbey was located and a Stone Cross in the nearby Churchyard, the Stone Cross being viewable from the park. The memorial states that it is constructed with stone brought from near his birthplace in France. Nowadays Simon is known as the father of the House of Commons.