Simon de Montfort, 5th Earl of Leicester, also Simon IV de Montfort (1160 – June 25, 1218) was a French nobleman who took part in the Fourth Crusade (1202 - 1204) and was a prominent leader of the Albigensian Crusade. He died at the siege of Toulouse in 1218.
He was the son of Simon III de Montfort , descended from the lords of Montfort l'Amaury in Normandy, and Amicia de Beaumont. He succeeded his father as Baron de Montfort in 1181; in 1190 he married Alice de Montmorency , the daughter of Bouchard III de Montmorency .
In 1199, while taking part in a tournament at Ecry-sur-Aisne , he heard Fulk of Neuilly preaching the crusade, and in the company of Count Thibaud de Champagne, he took the cross. The crusade soon fell under Venetian control, and was diverted to Zara on the Adriatic Sea. Pope Innocent III had specifically warned the Crusaders not to attack fellow Christians; Simon tried to reassure the citizens of Zara that there would be no attack, but nevertheless, the city was sacked in 1202. Simon did not participate in this action, and soon he left the Crusade altogether. Afterwards, under Venetian guidance the Crusaders sacked the city of Constantinople—the main trading rival to Venice.
His mother was the eldest daughter of Robert de Beaumont, 3rd Earl of Leicester. After the death of her brother Robert de Beaumont, 4th Earl of Leicester without children in 1204, she inherited half of his estates, and a claim to the Earldom of Leicester. The division of the estates was effected early in 1207, by which the rights to the earldom were assigned to Amicia and Simon. However, King John of England took possession of the lands himself in February 1207. Later, in 1215, the lands were passed into the hands of Simon's nephew, Ranulph de Meschines, 4th Earl of Chester .
Simon remained on his estates in France, where in 1208 he was made captain-general of the French forces in the Albigensian Crusade. Simon was given the territory conquered from Raymond VI of Toulouse. He became notorious and feared for his extreme cruelty, massacring whole towns, and for his "treachery, harshness, and bad faith." In 1213 he defeated Peter of Aragon at the Battle of Muret . The Albigensians were now crushed, but Simon carried on the campaign as a war of conquest, being appointed lord over all the newly-acquired territory as Count of Toulouse and Duke of Narbonne (1215). He occupied himself in waging war at Nîmes, until in 1217 a rebellion broke out in Provence, where Count Raymond's son re-entered Toulouse. Simon hastened to besiege the city, and was killed on 25 June 1218 while fighting a sally by the besiegers. He was buried in the Monastery of Haute-Bruyère.
Simon left three sons: his French estates passed to his eldest son, Amaury de Montfort, while his younger son, Simon, eventually gained possession of the earldom of Leicester and played a major role in the reign of Henry III of England.