Valdemar IV Atterdag (the epithet meaning "A New Dawn") or Waldemar (c. 1320 – 24 October 1375); Danish: Valdemar Atterdag, was King of Denmark from 1340 to 1375.

He was the youngest son of Christopher II and spent most of his childhood and youth in exile at the court of Louis IV, Holy Roman Emperor in Bavaria after the defeats of his father and the death and imprisonment, respectively, of his two older brothers Eric and Otto at the hand of the Holsteiners. Here he acted as a pretender waiting for a comeback.

Following the assassination of Count Gerhard III by Niels Ebbesen and his brothers, Valdemar was proclaimed King of Denmark at the Viborg Assembly (landsting) on St Hans Day, 24 June 1340 led by Niels Ebbesen. By his marriage with Helvig, the daughter of Eric II, Duke of Schleswig, and with what was left to him by his father, he controlled about one quarter of the territory of Jutland north of the Kongeå river.

He was not compelled to sign a charter as his father had done, probably because Denmark had been without a king for years, and no one expected the twenty-year-old king to be any more trouble to the great nobles than his father had been. But Valdemar was a clever and determined man and realized that the only way to rule Denmark was to get control of its territory.

Ebbesen attempted to liberate central Jutland from the Holsteiners at the siege of Sønderborg Castle on 2 November 1340, but Ebbesen and his brothers were killed.

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