Baldwin I of Flanders (died 879), known as Iron Arm, was the first count of Flanders.
Baldwin was the son of Odacre, who had been created first count of Flanders in 862. He rose to prominence in 863, when he eloped with Judith, daughter of Charles the Bald. (Judith had previously been married to Ethelwulf and his son Ethelbald, kings of Wessex.) Charles was not pleased, and he had Baldwin excommunicated. Baldwin responded by traveling to Rome to plead his case to Pope Nicholas I. When this was granted Charles relented and accepted the marriage. To give Baldwin proper status, Charles made him count of Ghent. In the following years Baldwin was also given the counties of Ternois and Flanders.
Baldwin was the son of Baldwin V of Hainaut, and Margaret I, sister of Philip of Alsace and Countess of Flanders.
Baldwin took possession of a much-reduced Flanders, for his uncle had given a large chunk, including Artois, as dowry to Baldwin's sister Isabelle of Hainaut on her marriage to King Philip II of France, and another significant piece to his own wife.
Baldwin's uncle William of Thy (an illegitimate son of Baldwin IV of Hainaut) was regent for Hainaut.