Baldwin I (probably 830s – 879), also known as Baldwin Iron Arm (the epithet is first recorded in the 12th century), was the first Margrave of Flanders.

At the time Baldwin first appears in the records he was already a count, presumably in the area of Flanders, but this is not known. Count Baldwin rose to prominence when he eloped with princess Judith, daughter of Charles the Bald, king of West Francia. Judith had previously been married to Æthelwulf and his son (from an earlier marriage) Æthelbald, kings of Wessex, but after the latter's death in 860, she returned to France.

Around the Christmas of 861, at the instigation of Baldwin and with her brother Louis's consent, Judith escaped the custody into which she had been placed in the city of Senlis, Oise after her return from England. She fled north with Count Baldwin. Charles had given no permission for a marriage and tried to capture Baldwin, sending letters to Rorik of Dorestad and Bishop Hungar, forbidding them to shelter the fugitive.

After Baldwin and Judith had evaded his attempts to capture them, Charles had his bishops excommunicate the couple. Judith and Baldwin responded by travelling to Rome to plead their case with Pope Nicholas I. Their plea was successful and Charles was forced to accept the situation. The marriage took place on 13 December 862 in Auxerre. By 870, Baldwin had acquired the lay-abbacy of Saint Peter's Abbey in Ghent and is assumed to have also acquired the counties of Flanders and Waasland, or parts thereof by this time.

Baldwin developed himself as a very faithful and stout supporter of Charles and played an important role in the continuing wars against the Vikings. He is named in 877 as one of those willing to support the emperor's son, Louis the Stammerer. During his life, Baldwin expanded his territory into one of the major principalities of Western Francia. He died in 879 and was buried in the Abbey of St-Bertin, near Saint-Omer.

Baldwin was succeeded by his and Judith's son, Baldwin II (c. 866 – 918). The couple's first son, Charles, named after his maternal grandfather, died at a young age. His third son Raoul (Rodulf) (c. 869 – murdered 896) became Count of Cambrai around 888, but he and his brother joined king Zwentibold of Lotharingia in 895. In 896, they attacked Vermandois and captured Arras, Saint-Quentin and Peronne, but later that year Raoul was captured by Count Herbert and killed.