James I of Aragon (Catalan: Jaume I, Spanish: Jaime I) (Montpellier February 2, 1208 – July 27, 1276), surnamed the Conqueror, was the king of Aragon, count of Barcelona and Lord of Montpellier from 1213 to 1276.

He was the only child of Peter II of Aragon and Marie of Montpellier. As a child he was a pawn of power politics in Provence, where his father was engaged in struggles in the wars between the Cathars of Albi and Simon de Montfort. Peter endeavoured to placate the northern crusaders by arranging a marriage between his son James and Simon's daughter, entrusting the boy to be educated in Montfort's care in 1211, but Peter was soon forced to take up arms against them, and he was slain at the Battle of Muret September 12, 1213. Montfort would willingly have used James as a means of extending his own power. The Aragonese and Catalans, however, appealed to the pope, who forced Montfort to surrender him in May or June 1214.

James was now entrusted to the care of Guillen de Monredon, the head of the Knights Templar in Spain and Provence. The kingdom was given over to confusion till in 1216 the Templars and some of the more loyal nobles brought the young king to Zaragoza.

After a false start at uniting Aragon with Navarre through a scheme of mutual adoption, James turned to the south and the Mediterranean, conquered the Balearic Islands (from 1228 over the following four years) and Valencia (the city capitulated September 28, 1238).

With the French, James endeavoured to form a state straddling the Pyrenees, to counterbalance the power of France north of the Loire. As with the earlier Visigothic attempt, this policy was victim of 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, with Louis IX, signed May, 1258, he frankly withdrew from conflict with the French king, and was content with the recognition of his position, and the surrender of antiquated and illusory French claims to the overlordship of Catalonia.

During his remaining two decades, James warred with the Moors in Murcia, on behalf of his son-in-law Alphonso the Wise of Castile. As a legislator and organizer he occupies a high place among the Spanish kings. The favor he showed his illegitimate offspring led to protest from the nobles, and to conflicts between his sons legitimate and illegitimate. When one of the latter, Fernan Sanchez, 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.

At the close of his life King James divided his states between his sons by Yolande of Hungary, Peter receiving the Hispanic possessions on the mainland and James, the Kingdom of Majorca (the Balearic Islands and the counties of Roussillon and Cerdanya) and the Lordship of Montpellier, a division which inevitably produced fratricidal conflicts. The king fell very ill at Alzira, and resigned his crown, intending to retire to the monastery of Poblet, but died at Valencia on July 7, 1276.

King James wrote or dictated at various stages a chronicle of his own life, "Llibre Dels Feits" 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, the growth of national sentiment based on homeland, language and culture, and medieval military tactics.

Marriage and children
He first married, in 1221, Leonor, daughter of Alfonso VIII of Castile, and then after having the marriage annulled (though a son was declared legitimate), in 1235, Yolande (Violant) of Hungary, daughter of Andrew II of Hungary. He had several lovers, both during and after his marriage, and a few bore him illegitimate sons. He married thirdly Teresa Gil de Vidaure, but only by a private document, and left her when she developed leprosy.
By Leonor of Castile:
1. Alfonso (1229-1260), married Constance of Moncada, Countess of Bigorre

By Violant of Hungary:
1. Violant of Aragon (1236-1301), married Alfonso X of Castile
2. Constance of Aragon (1239-1269), married Juan Manuel of Castile, son of Ferdinand III of Castile
3. Peter III of Aragon (1240-1285)
4. James II of Majorca (1243-1311)
5. Ferdinand of Aragon (1245-1250)
6. Sança of Aragon (1246-1251)
7. Isabella of Aragon (1247-1271), married Philip III of France
8. Maria of Aragon (1248-1267), nun.
9. Sancho, Archbishop of Toledo (1250-1279)
10. Elionor of Aragon (1251-?, young)

By Teresa Gil de Vidaure:
1. Jaume of Xèrica (c. 1255-1285), lord of Xèrica
2. Pere (1259-1318), lord of Ayerbe

Illegitimate by Blanca d'Antillón:
• Ferran Sanchis or Fernando Sánchez de Castro (1240), baron of Castro, killed in 1275 by his half-brother Peter III.
Illegitimate by Berenguela Fernández:
• Pedro Fernández, baron of Híjar
Illegitimate by Elvira Sarroca:
• Jaume Sarroca (1248), Archbishop of Huesca