Valdemar Atterdag (c. 1320 - October 24, 1375) was a King of Denmark (1340 - 1375). He was the youngest son of Christopher II.

He was elected king in 1340 following the assassination of Count Gerhard III , although his territory at this time was limited to the northernmost county of Jutland. He gradually reacquired the rest of Jutland and Zealand. He finally reunited Denmark by capturing Scania and the island of Gotland in 1361. However, the resulting conflict with the Hanseatic League ended in a Danish defeat, and in 1370 he was forced to submit to the Treaty of Stralsund.

He was succeeded by his grandson Oluf III of Denmark, the offspring of his daughter Margaret and Haakon VI of Norway, son to Magnus II of Sweden.