Sweyn III Grathe (Danish: Svend III Grathe) (c. 1125 – 23 October 1157) was the King of Denmark between 1146 and 1157, in shifting alliances with Canute V and his own cousin Valdemar I. In 1157, the three agreed a tripartition of Denmark. Sweyn attempted to kill his rivals at the peace banquet, and was subsequently defeated by Valdemar I at the Battle of Grathe Heath and killed.

Sweyn was the illegitimate son of Erik II the Memorable and the concubine Thunna. Sweyn travelled with Eric II to Norway in the mid-1130s, when his father fought King Niels to win the Danish throne. When Eric II died in 1137, he was succeeded by Eric III, and Sweyn was sent to the court of Conrad III of Germany. Here he befriended Conrad's nephew Frederick, the later Friedrick I of Germany.

He travelled to Denmark, where he and his cousin Valdemar (the later Valdemar I of Denmark) sought to canonize Sweyn's uncle and Valdemar's father Canute Lavard in 1146, under protest from Archbishop Eskil of Lund in Scania. At the abdication of Eric III in 1146, Sweyn was elected king by the magnates on Zealand while Canute V was crowned by their counterparts in Jutland.

For the next years, Sweyn fought a civil war against Canute for the kingship of Denmark, supported by Valdemar. Canute was supported by Archbishop Eskil, but Sweyn moved to secure Eskil's loyalty by granting the Archbishop of Lund land holdings in Scania and Bornholm. Sweyn subsequently defeated Canute on Zealand, to confine him to Jutland. In 1147, Sweyn and Canute united to support the Wendish Crusade. As Sweyn engaged the Wends in naval battle, he received little support from Canute, and lost his flagship. The civil war was soon re-ignited.

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