Sweyn III Grathe (Danish: Svend III Grathe) (c. 1125 – 23 October 1157) is the King of Denmark between 1146 and 1157, in shifting alliances with Canute V and his own cousin Valdemar I.

In 1157, the three agree to a tripartition of Denmark.

Sweyn attempts to kill his rivals at the peace banquet, and is subsequently defeated by Valdemar I at the Battle of Grathe Heath and killed.