Mu'izz ad-Din Ahmed Sanjar (1084/1086 - May 8 1157) was the sultan of Great Seljuk from 1118 to 1153.

He was a son of Malik Shah I and participated in the succession wars against his three brothers and one uncle, namely Mahmud I , Barkiyaruq , Malik Shah II and Mehmed I. From 1097, Ahmed Sanjar was the ruler of most of Persia, his capital being situated in Khorasan. A number of rulers revolted against him and the Great Seljuk Empire split.

Ahmed Sanjar was captured by Turkmen nomads in 1153 and was held captive until 1156. He died the following year.