Baldwin I., prince of Edessa (1098-1100), and first king of Jerusalem (1100-1118), was the brother of Godfrey of Bouillon (q.v.). He was originally a clerk in orders, and held several prebends; but in 1096 he joined the first crusade, and accompanied his brother Godfrey as far as Heraclea in Asia Minor. When Tancred left the main body of the crusaders at Heraclea, and marched into Cilicia, Baldwin followed, partly in jealousy, partly from the same political motives which animated Tancred. He wrested Tarsus from Tancred's grip (September 1097), and left there a garrison of his own. After rejoining the main army at Marash, he received an invitation from an Armenian named Pakrad, and moved eastwards towards the Euphrates, where he occupied Tell-bashir. Another invitation followed from Thoros of Edessa; and to Edessa Baldwin came, first as protector, and then, when Thoros was assassinated, as his successor (March 1098). For two years he ruled in Edessa (1098-1100), marrying an Armenian wife, and acting generally as the intermediary between the crusaders and the Armenians. During these two years he was successful in maintaining his ground, both against the Mahommedan powers by which he was surrounded, and from which he won Samosata and Seruj (Sarorgia), and against a conspiracy of his own subjects in 1098. At the end of 1099 he visited Jerusalem along with Bohemund I.; but he returned to Edessa in January 1100. On the death of Godfrey he was summoned by a party in Jerusalem to succeed to his brother.
A lay reaction against the theocratic pretensions of Dagobert, who was counting on Norman support, was responsible for the summons; and in the strength of that reaction Baldwin was able to become the first king of Jerusalem. He was crowned on Christmas Day, 1100, by the patriarch himself; but the struggle of church and state was not yet over, and in the spring of 1101 Baldwin had Dagobert suspended by a papal legate, while later in the year the two disagreed on the question of the contribution to be made by the patriarch towards the defence of the Holy Land. The struggle ended in the deposition of Dagobert and the triumph of Baldwin (1102).
As Baldwin had secured the supremacy of the lay power in Jerusalem, so he extended into a compact kingdom the poor and straggling territories to which he had succeeded. This he did by an alliance with the Italian trading towns, especially Genoa, which supplied in return for the concession of a quarter in the conquered towns, the instruments and the skill for a war of sieges, in which the coast towns of Palestine were successively reduced. Arsuf and Caesarea were captured in 1101; Acre in 1104; Beirut and Sidon in 1110 (the latter with the aid of the Venetians and Norwegians). Meanwhile Baldwin repelled in successive years the attacks of the Egyptians (1102, 1103, 1105), and in the latter years of his reign (1115-1118) he even pushed southward at the expense of Egypt, penetrating as far as the Red Sea, and planting an outpost at Monreal. In the north he had to compose the dissensions of the Christian princes in Tripoli, Antioch and Edessa (1109-1110), and to help them to maintain their ground against the Mahommedan princes of N.E. Syria, especially Maudud and Aksunk-ur, amirs of Mosul. In this way Baldwin was able to make himself into practical suzerain of the three Christian principalities of the north, though the suzerainty was, and always continued to be, somewhat nominal.
In 1118 he died, after an expedition to Egypt, during which he captured Farama, and, as old Fuller says, "caught many fish, and his death in eating them."
Baldwin was one of the "adventurer princes" of the first crusade, and as such he stands alongside of Bohemund, Tancred and Raymund. On the whole he was the most successful of his class. By his defence of the lay power against a nascent theocracy, and by his alliance with the Italian towns, he was the real founder of the Latin kingdom of Jerusalem. Events worked for him: he might never have come to the throne, unless Bohemund had fallen into the hands of Danishmend; and the dissensions among the Mahommedans alone made possible the subsequent consolidation of his kingdom. But he had virtù as well as fortuna; and on his tombstone it was written that he was "a second Judas Maccabaeus, whom Kedar and Egypt, Dan and Damascus dreaded." As king, he still retained something of the clerk in the habit of his dress; but he was at the same time a warrior so impetuous, as to be sometimes foolhardy, and his policy was on the whole anti-clerical. He may be accused of greed: his life was not chaste; and the two defects met in his rejection of his Armenian wife and his marriage to the rich Sicilian widow Adelaide (1113). But "on the holiest soil of history, he gave his people a fatherland"; and Fulcher of Chartres, his chaplain, who paints at the beginning of Baldwin's reign the terrors of the lonely band of Christians in the midst of their foes, can celebrate at the end the formation of a new nation in the East (qui fuimus occidentales, nunc facti sumus orientales) - an achievement which, so far as it was the work of any one man, was the work of Baldwin I.
The Historia Hierosolymitana of Fulcher, who had accompanied Baldwin as chaplain to Edessa, and had lived in Jerusalem during his reign, is the primary authority for Baldwin's career. There is a monograph on Baldwin by Wolff (König Baldwin I. von Jerusalem), and his reign is sketched in R. Röhricht's Geschichte des Königreichs Jerusalem (Innsbruck, 1898) C. i.-iv.