Baldwin I (1060s – 2 April 1118) was the first count of Edessa from 1098 to 1100 and king of Jerusalem from 1100 to his death in 1118. He was the youngest son of Eustace II, Count of Boulogne, and Ida of Lorraine and married a Norman noblewoman, Godehilde of Tosny. He received the County of Verdun in 1096, but he soon joined the crusader army of his brother Godfrey of Bouillon and became one of the most successful commanders of the First Crusade.

While the main crusader army was marching across Asia Minor in 1097, Baldwin and the Norman Tancred launched a separate expedition against Cilicia. Tancred tried to capture Tarsus in September, but Baldwin forced him to leave it, which gave rise to an enduring conflict between them. Baldwin seized important fortresses in the lands to the west of the Euphrates with the assistance of local Armenians. Thoros of Edessa invited him to come to Edessa to fight against the Seljuks. Taking advantage of a riot against Thoros, Baldwin seized the town and established the first Crusader state on 10 March 1098. To strengthen his rule, the widowed Baldwin married an Armenian ruler's daughter (who is now known as Arda). He supplied the main crusader army with food during the siege of Antioch. He defended Edessa against Kerbogha, the governor of Mosul, for three weeks, preventing him from reaching Antioch before the crusaders captured it.

Godfrey of Bouillon, whom the crusaders had elected their first ruler in Jerusalem, died in 1100. Daimbert, the Latin patriarch, and Tancred offered Jerusalem to Tancred's uncle, Bohemond I of Antioch. Godfrey's retainers took possession of the town and urged Baldwin to claim Godfrey's inheritance. Since a Muslim ruler had captured Bohemond, Baldwin marched to Jerusalem, meeting little resistance. The patriarch crowned him king in Bethlehem on 25 December. He captured Arsuf and Caesarea in 1101, Acre in 1104, Beirut in 1110, and Sidon in 1111, with the assistance of Genoese and Venetian fleets and of several smaller crusader groups, but all his attempts to capture Ascalon and Tyre failed. After his victory at the third battle of Ramla in 1105, the Egyptians launched no further major campaigns against the Kingdom of Jerusalem.

Baldwin helped Bertrand, Count of Toulouse, to capture Tripoli in 1109. Being the only crowned monarch in the Latin East, Baldwin claimed suzerainty over other crusader rulers. Baldwin II of Edessa and Bertrand swore fealty to him. Tancred, who ruled the Principality of Antioch, also obeyed his summons. Baldwin supported Baldwin II and Tancred against Kerbogha's successor, Mawdud, who launched a series of campaigns against Edessa and Antioch in the early 1110s. He erected fortresses in Oultrejordain—the territory to the east of the Jordan River—to control the caravan routes between Syria and Egypt. He died during a campaign against Egypt in 1118.

Born some time after 1060, Baldwin was the third son of Count Eustace II of Boulogne and Ida of Lorraine. Being his parents' youngest son, he was intended for a career in the Church. He studied the liberal arts and held prebends in the cathedrals of Cambrai, Rheims and Liège. For reasons that are unknown, and at an unspecified time, he abandoned his church career and became a knight. The historian John France says that Baldwin most probably realised that the Gregorian Reform had diminished his chance to seize rich benefices. Historian Susan B. Edgington, on the other hand, proposes that Baldwin preferred a secular career because his childless brother, Godfrey of Bouillon, duke of Lower Lotharingia, had taken ill suddenly, giving Baldwin a chance to inherit his duchy.

Baldwin married a Norman noblewoman, Godehilde of Tosny, whose family owned land and property in both Normandy and England. Baldwin and his wife most probably settled in the court of his eldest brother, Count Eustace III of Boulogne. Eustace and Baldwin jointly fought for their brother, Godfrey, against Count Albert III of Namur and Bishop Theoderic of Verdun at Stenay in 1086. Godfrey mentioned Baldwin in most of his charters of grant, indicating that Baldwin was regarded as his designated heir. Baldwin regularly visited the fortress of his wife's family at Conches-en-Ouche.

Pope Urban II proclaimed the First Crusade at the Council of Clermont on 27 November 1095. Godfrey of Bouillon decided to join the military campaign and sold or mortgaged his inherited domains to raise funds. One of his domains, the County of Verdun, was seized by Richer, Bishop of Verdun, who soon granted it to Baldwin. The dissolution of Godfrey's allodial lands deprived all future dukes of the basis of their authority in Lower Lotharingia, which facilitated Baldwin's decision to take the Cross. Eustace III of Boulogne also joined the crusade. According to a letter from Pope Urban, only the army that Peter the Hermit had mustered for the People's Crusade outnumbered the three brothers' force.

Baldwin departed for the crusade with Godfrey's army on 15 August 1096. His wife and children accompanied him, suggesting that he had decided not to return to his homeland. The crusaders stopped at Tulln an der Donau before reaching the frontier of Hungary in September. Godfrey left Baldwin in charge of his troops during his conference with Coloman of Hungary, to discuss the conditions of the crusaders' march across the country. He agreed to hand over Baldwin, along with Baldwin's wife and retainers, as hostages, to ensure their troops' good conduct. Baldwin and Godehilde were released soon after the crusaders left Hungary. They entered the Byzantine Empire near Belgrade in late November.