Baldwin I of Flanders (d. 879), also known as Baldwin Iron Arm, was the first count of Flanders.

Baldwin was the son of Odacre, and had been created first count of Flanders in 862. Baldwin rose to prominence 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 succeeded by his son by Judith, Baldwin II of Flanders.

Through his descendants Matilda of Flanders and Henry I of England, he was an ancestor to the present-day British royal family including Elizabeth II of the United Kingdom of Great Britain and Northern Ireland and her son Charles, Prince of Wales.