Margaret I (died 15 November 1194) was countess of Flanders suo jure from 1191 to her death. She was the daughter of Thierry, Count of Flanders, and Sibylla of Anjou, and the heiress of her childless brother, Philip of Flanders.In 1160 she married Ralph II, Count of Vermandois. He died of leprosy in 1167 without issue.In 1169 she married Baldwin V, Count of Hainaut, her third cousin once removed, who became her co-ruler when she gained the county of Flanders in 1191.