Thierry of Alsace (Dietrich) (c. 1099 – January 17, 1168), in Flanders known as Diederik van den Elzas, is count of Flanders from 1128 to 1168.

He is the youngest son of Duke Thierry II of Lorraine and Gertrude of Flanders (daughter of Robert I of Flanders).

With a record of four campaigns in the Levant and Africa (including participation in the Second Crusade, the failed 1157–1158 siege of the Syrian city Shaizar, and the 1164 invasion of Egypt), he has a rare and distinguished record of commitment to crusading.