Margaret, often called Margaret of Constantinople, ruled as Countess of Flanders during 1244–1278 and Countess of Hainaut during 1244–1253 and 1257–1280. She was the younger daughter of Baldwin IX, Count of Flanders and Hainaut, and Marie of Champagne.

The count of Flanders was the ruler or sub-ruler of the county of Flanders, beginning in the 9th century. Later, the title would be held for a time, by the rulers of the Holy Roman Empire and Spain. During the French Revolution, in 1790, the county of Flanders was annexed to France and ceased to exist. In the 19th century, the title was appropriated by Belgium and granted twice to younger sons of Belgian kings. The most recent holder died in 1983.