James I the Conqueror (Catalan: Jaume el Conqueridor, Aragonese: Chaime lo Conqueridor, Spanish: Jaime el Conquistador, Occitan: Jacme lo Conquistaire; 2 February 1208 – 27 July 1276) was the King of Aragon, Count of Barcelona, and Lord of Montpellier from 1213 to 1276. His long reign saw the expansion of the Crown of Aragon on all sides: into Valencia to the south, Languedoc to the north, and the Balearic Islands to the east. By a treaty with Louis IX of France, he wrested the county of Barcelona from nominal French suzerainty and integrated it into his crown. His part in the Reconquista was similar in Mediterranean Spain to that of his contemporary Ferdinand III of Castile in Andalusia.

As a legislator and organiser, he occupies a significant place among the Spanish kings. James compiled the Libre del Consulat de Mar, which governed maritime trade and helped establish Aragonese supremacy in the western Mediterranean. He was an important figure in the development of Catalan, sponsoring Catalan literature and writing a quasi-autobiographical chronicle of his reign: the Llibre dels fets.

James was born at Montpellier as the only son of Peter II and Mary, heiress of William VIII of Montpellier and Eudokia Komnene. As a child, James was a pawn in the power politics of Provence, where his father was engaged in struggles helping the Cathar heretics of Albi against the Albigensian Crusaders led by Simon IV de Montfort, Earl of Leicester, who were trying to exterminate them. Peter endeavoured to placate the northern crusaders by arranging a marriage between his son James and Simon's daughter. He entrusted the boy to be educated in Montfort's care in 1211, but was soon forced to take up arms against him, dying at the Battle of Muret on 12 September 1213. Montfort would willingly have used James as a means of extending his own power had not the Aragonese and Catalans appealed to Pope Innocent III, who insisted that Montfort surrender him. James was handed over, at Carcassonne, in May or June 1214, to the papal legate Peter of Benevento. James was then sent to Monzón, where he was entrusted to the care of William of Montredon, the head of the Knights Templar in Spain and Provence; the regency meanwhile fell to his great uncle Sancho, Count of Roussillon, and his son, the king's cousin, Nuño. The kingdom was given over to confusion until, in 1217, the Templars and some of the more loyal nobles brought the young king to Zaragoza.

In 1221, he was married to Eleanor, daughter of Alfonso VIII of Castile and Leonora of England. The next six years of his reign were full of rebellions on the part of the nobles. By the Peace of Alcalá of 31 March 1227, the nobles and the king came to terms. In 1228, James faced the sternest opposition from a vassal yet. Guerau IV de Cabrera had occupied the County of Urgell in opposition to Aurembiax, the heiress of Ermengol VIII, who had died without sons in 1208. While Aurembiax's mother, Elvira, had made herself a protegée of James's father, on her death (1220), Guerao had occupied the county and displaced Aurembiax, claiming that a woman could not inherit. James intervened on behalf of Aurembiax, whom he owed protection. He bought Guerau off and allowed Aurembiax to reclaim her territory, which she did at Lleida, probably also becoming one of James' earliest mistresses. She surrendered Lleida to James and agreed to hold Urgell in fief from him. On her death in 1231, James exchanged the Balearic Islands for Urgell with her widower, Peter of Portugal.

From 1230 to 1232, James negotiated with Sancho VII of Navarre, who desired his help against his nephew and closest living male relative, Theobald IV of Champagne. James and Sancho negotiated a treaty whereby James would inherit Navarre on the old Sancho's death, but when this did occur, the Navarrese nobless instead elevated Theobald to the throne (1234), and James disputed it. Pope Gregory IX was required to intervene. In the end, James accepted Theobald's succession.

James endeavoured to form a state straddling the Pyrenees, to counterbalance the power of France north of the Loire. As with the much earlier Visigothic attempt, this policy was victim to physical, cultural, and political obstacles. As in the case of Navarre, he was too wise to launch into perilous adventures. By the Treaty of Corbeil, signed in May 1258, he frankly withdrew from conflict with Louis IX of France and was content with the recognition of his position, and the surrender of antiquated and illusory French claims to the overlordship of Catalonia.

After his false start at uniting Aragon with the Kingdom of Navarre through a scheme of mutual adoption, James turned to the south and the Mediterranean Sea, where he conquered Majorca on 10 September in 1229 and the rest of the Balearic Islands; Minorca 1232; Ibiza1235) and where Valencia capitulated 28 September 1238.

During his remaining two decades after Corbeil, James warred with the Moors in Murcia, on behalf of his son-in-law Alfonso X of Castile. On 26 March 1244, the two monarchs signed the Treaty of Almizra to determine the zones of their expansion into Andalusia so as to prevent squabbling between them. Specifically, it defined the borders of the newly-created Kingdom of Valencia. James signed it on that date, but Alfonso did not affirm it until much later. According to the treaty, all lands south of a line from Biar to Villajoyosa through Busot were reserved for Castile.

The "khan of Tartary" (actually the Ilkhan) Abaqa corresponded with James in early 1267, inviting him to join forces with the Mongols and go on Crusade. James sent an ambassador to Abaqa in the person of Jayme Alaric de Perpignan, who returned with a Mongol embassy in 1269. Pope Clement IV tried to dissuade James from Crusading, regarding his moral character as sub-par, and Alfonso X did the same. Nonetheless, James, who was then campaigning in Murcia, made peace with Mohammed I ibn Nasr, the Sultan of Granada, and set about collecting funds for a Crusade. After organising the government for his absence and assembling a fleet at Barcelona in September 1269, he was ready to sail east. A storm, however, drove him off course and he landed at Aigues-Mortes. According to the continuator of William of Tyre, he returned via Montpellier por l'amor de sa dame Berenguiere ("for the love his lady Berengaria") and abandoned any further effort at a Crusade. James' bastard sons Pedro Fernández and Fernán Sánchez, who had been given command of part of the fleet, did continue on their way to Acre, where they arrived in December. They found that Baibars, the Mamluk sultan of Egypt, had broken his truce with the Kingdom of Jerusalem and was making a demonstration of his military power in front of Acre. During the demonstration, Egyptian troops hidden in the bushes ambushed a returning Frankish force which had been in Galilee. James' sons, initially eager for a fight, changed their minds after this spectacle and returned home via Sicily, where Fernán Sánchez was knighted by Charles of Anjou

James built and consecrated the Cathedral of Lleida, which was constructed in a style transitional between Romanesque and Gothic with little influence from Moorish styles.
James was a patron of the University of Montpellier, which owed much of its development to his impetus. He also founded a studium at Valencia in 1245 and received privileges for it from Pope Innocent IV, but it did not develop as splendidly. In 1263, James presided over a debate in Barcelona between the Jewish rabbi Nahmanides and Pablo Christiani, a prominent converso. James was the first great sponsor and patron of vernacular Catalan literature. Indeed, he may himself be called "the first of the Catalan prose writers."^. James wrote or dictated at various stages a chronicle of his own life, Llibre dels fets in Catalan, which is the first self-chronicle of a Christian king. As well as a fine example of autobiography the "Book of Deeds" expresses concepts of the power and purpose of monarchy; examples of loyalty and treachery in the feudal order; and medieval military tactics. More controversially, some historians have looked at these writings as a source of Catalan identity, separate from that of Occitania and Rome. James also wrote the Libre de la Saviesa or "Book of Wisdom". The book contains proverbs from various authors going back as far as King Solomon and as close to his own time, such as Albert the Great. It even contains maxims from the medieval Arab philosophers and from the Apophthegmata Philosophorum of Honein ben Ishak, which was probably translated at Barcelona during his reign. A Hebrew translator by the name of Jehuda was employed at James's court during this period. Though James was himself a prose writer and sponsored mostly prose works, he had an appreciation of verse. In consequence of the Albigensian Crusade, many troubadours were forced to flee southern France and many found refuge in Aragon. Notwithstanding his early patronage of poetry, by the influence of his confessor Ramon de Penyafort, James brought the Inquisition into his realm in 1233 to prevent any vernacular translation of the Bible.

The favour James showed his illegitimate offspring led to protest from the nobles, and to conflicts between his sons legitimate and illegitimate. When one of the latter, Fernán Sánchez, who had behaved with gross ingratitude and treason to his father, was slain by the legitimate son Peter, the old king recorded his grim satisfaction.

In his Will James divided his states between his sons by Yolanda of Hungary: the aforementioned Peter received the Hispanic possessions on the mainland and James, the Kingdom of Majorca (including the Balearic Islands and the counties of Roussillon and Cerdanya) and the Lordship of Montpellier. The division inevitably produced fratricidal conflicts. Always the home de fembres (“lady’s man”), he eloped with the wife of one of his vassals in his final years and was excommunicated for his efforts by Pope Gregory X. In 1276, the king fell very ill at Alzira and resigned his crown, intending to retire to the monastery of Poblet, but he died at Valencia on 27 July.