Baldwin of Boulogne (died 1118), count of Edessa (1098—1100), and first king of Jerusalem (1100—1118), was the brother of Godfrey of Bouillon, son of Eustace II of Boulogne.

As the younger brother of Godfrey and Eustace, Baldwin was originally intended for a career in the church, but he had given this up around 1080. Afterwards he lived in Normandy, where he married into a noble family, but returned to Lorraine in order to take control of the county of Verdun (previously held by Godfrey).

In 1096 he joined the First Crusade, along with his wife and his brothers, selling much of his property to the church in order to pay for his expenses. During the march he served as a hostage for Coloman of Hungary, ensuring that the crusaders would not pillage Hungarian territory. He accompanied his brother Godfrey as far as Heraclea in Asia Minor, where he broke away from the main body of the crusaders with Tancred to march into Cilicia. Tancred was surely seeking to capture some land and establish himself as a petty ruler in the east, and Baldwin may have had the same goal.

In September of 1097 he took Tarsus from Tancred, and installed his own garrison in the city, with help from a fleet of pirates from Boulogne. Tancred and Baldwin's armies skirmished briefly at Mamistra, but the two never came to open warfare and Tancred marched on towards Antioch. After rejoining the main army at Marash , and learning that his wife had died in the meantime, Baldwin received an invitation from an Armenian named Pakrad, and moved eastwards towards the Euphrates where he occupied Turbessel . Another invitation came from Thoros of Edessa, where Baldwin was adopted as Thoros' son and successor. When Thoros was assassinated in March of 1098, Baldwin became the first count of Edessa, although it is unknown if he played any role in the assassination. He ruled the county until 1100, marrying an Armenian wife, and acting as an ambassador between the crusaders and Armenians.

During these two years he captured Samosata and Seruj (Sarorgia ) from the Muslims, and defeated a conspiracy by some of his Armenian subjects in 1098. At the end of 1099 he visited Jerusalem along with Bohemund I of Antioch, but he returned to Edessa in January 1100. After Godfrey's death in 1100 he was invited to Jerusalem by the supporters of a secular monarchy, and was crowned as the first king of Jerusalem on Christmas Day, 1100, by Dagobert of Pisa, the Patriarch, who would have preferred to turn the kingdom into a theocracy. 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 in 1102.

After Baldwin had secured the supremacy of the monarchy in Jerusalem, he extended the influence of the kingdom over the other cities that had not yet been captured. He was helped by an alliance with the Italian trading towns, especially Genoa, which supplied siege engines and naval support in return for the trading quarters in each of the conquered cities.

The most powerful enemy during Baldwin's reign was Egypt. He defeated Egyptian invasions in 1102, 1103, and 1105, and from 1115 to 1118 he captured territory from them, gaining access to the Red Sea and building the fortress of Montreal. To the north, he limited Damascene access to the Mediterranean coast by capturing Arsuf and Caesarea in 1101, Acre in 1104, and Beirut and Sidon in 1110 (the latter with the aid of the Venetians, and Norwegians under King Sigurd I).

Among the other Christian rulers, he became the nominal suzerain of the County of Tripoli, the Principality of Antioch and the County of Edessa, and helped them defend against Muslim invasions from Syria, especially Mawdud and Aksunk-ur of Mosul. In 1103 he paid the ransom for Bohemund of Antioch, who had been captured in battle; Baldwin preferred Bohemund to Tancred, who ruled Antioch as regent, and was also prince of Galilee earlier in Baldwin's reign. In 1109 he acted as arbitrator of a council of the greatest barons outside the walls of Tripoli, and forced Tancred to give up his claim to the city. Soon after the city fell to the crusaders.

Baldwin's personal life was controversial. In 1108 he forced his wife to enter a monastery in Jerusalem, accusing her of sexual relations with Muslim men. In 1113 he married Adelaide, the widow of Roger I of Sicily. By now it was suspected that Baldwin was Censored page, as he had no children. Adelaide and Roger I's son Roger II was supposed to become Baldwin's heir, but he was forced to give up the marriage in 1117, since his Armenian wife was still alive.

Baldwin died in 1118, after an expedition to Egypt, during which he captured Farama, and, as 17th century historian Thomas Fuller remarked, "caught many fish, and his death in eating them." His cousin Baldwin of Bourcq was chosen as successor, although the kingdom was also offered to Eustace, who did not want it.

The Historia Hierosolymitana of Fulcher of Chartres, who had accompanied Baldwin to Edessa as Baldwin's chaplain, and had lived in Jerusalem during his reign, is the primary source for Baldwin's career.

Preceded by: Godfrey	King of Jerusalem	Succeeded by: Baldwin II
—	Count of Edessa