Theoderic, commonly known as Thierry of Alsace, was the fifteenth count of Flanders from 1128 to 1168. With a record of four campaigns in the Levant and Africa, he had a rare and distinguished record of commitment to crusading.

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.