Pope Martin IV (between 1210 and 1220 – March 28, 1285), born Simon de Brion, held the papacy from February 21, 1281 until his death.

Simon de Brion, son of Jean, sieur de Brion, was born at the château of Meinpicien in the province of Touraine, France, in the decade following 1210. The seigneurial family de Brion, who took their name from Brion near Joigny, flourished in the Brie française. He spent time at the University of Paris, then reportedly studied law at Padua and Bologna. Through papal favour he received a canonry at St-Quentin, which he enjoyed in 1238, and spent a period 1248-1259 as a canon of the cathedral chapter in Rouen, finally as archdeacon. At the same time he was appointed treasurer of the church of St. Martin in Tours by Louis IX, an office he held until he was elected pope in 1281. In 1259, just as he disappears from the documents at Rouen, he was appointed to the council of the king, who made him keeper of the great seal, chancellor of France, one of the great officers in the household of the king.

In December 1261, the new French pope Urban IV made him cardinal-priest, with the titulus of the church of St. Cecilia. This entailed Simon de Brion's residence in Rome.

He returned to France as a legate for Urban IV and also for his successor Pope Clement IV, in 1264-1269 and again in 1274-1279, under Pope Gregory X. In the negotiations for papal support for the assumption of the crown of Sicily by Charles of Anjou, he became deeply politically entwined. As legate he presided over several synods on reform, the most important of which was held at Bourges in September, 1276.

Six months after the death of Pope Nicholas III in 1280, Charles of Anjou intervened in the papal conclave at Viterbo by imprisoning two influential Italian cardinals, on the grounds that they were interfering with the election. Without their opposition, Simon de Brie was unanimously elected to the papacy, taking the name Martin IV, on February 22, 1281.

Viterbo was placed under interdict for the imprisonment of the cardinals, and Rome was not at all inclined to accept a hated Frenchman as Pope, so Martin IV was crowned instead at Orvieto, on March 23, 1281.

Dependent on Charles of Anjou in nearly everything, the new Pope quickly appointed him to the position of Roman Senator. At the insistence of Charles, Martin IV excommunicated the Byzantine Emperor Michael VIII Palaeologus (1261-1282), who stood in the way of Charles' plans to restore the Latin Empire of the East that had been established in the aftermath of the Fourth Crusade. He thus broke the tenuous union which had been reached between the Greek and the Latin Churches at the Second Council of Lyons in 1274, and further compromise was rendered impossible.

In 1282, Charles was overthrown in the violent massacre known as the Sicilian Vespers. The Sicilians had elected Peter III of Aragon (1276-1285) as their King and sought papal confirmation in vain, though they were willing to reconfirm Sicily as a vassal state of the Papacy; Martin IV used all the spiritual and material resources at his command against the Aragonese, trying to preserve Sicily for the House of Anjou. He excommunicated Peter III, declared his kingdom of Aragon forfeit, and ordered a crusade against him, but it was all in vain.

With the death of his protector Charles d'Anjou, Martin was unable to remain at Rome. Pope Martin IV died at Perugia on March 28, 1285.

Among the seven cardinals created by Martin IV was Benedetto Gaetano, who afterwards ascended the papal throne as the famous Pope Boniface VIII (1294-1303).

In the Divine Comedy, Dante sees Martin IV in Purgatory, where the reader is reminded of the former pontiff's fondness for eels and wine.