Valdemar II (9 May 1170 or 28 June 1170 – 28 March 1241), called Valdemar the Victorious or Valdemar the Conqueror (Valdemar Sejr), was the King of Denmark from 1202 until his death in 1241. The nickname Sejr is a later invention and was not used during the King's own lifetime. Sejr means victory in Danish.

He was the second son of King Valdemar I and Sophia Valadarsdattir, a daughter of Richeza of Poland, and a West Russian Prince. When Valdemar's father died, young Valdemar was only twelve years old. He was named Duke of Southern Jutland (Latin: dux slesvicensis, literally Sleswickian duke), represented by the regent Bishop Valdemar Knudsen (1182–1193).

Bishop Valdemar was an ambitious man and disguised his own ambitions as young Valdemar's. When in 1192 Bishop Valdemar was named Prince-Archbishop of Bremen, his plot to overthrow King Canute VI with the help of German nobility and sit on Denmark's throne himself was revealed.

Duke Valdemar realized the threat Bishop Valdemar presented. He invited the archbishop to meet him in Aabenraa in 1192. Then the bishop fled to Swedish Norway to avoid arrest. The following year Bishop Valdemar organised - supported by the Hohenstaufens - a fleet of 35 ships and harried the coasts of Denmark, claiming the Danish throne for himself. In 1193 King Canute VI of Denmark captured him. Bishop Valdemar stayed in captivity in Nordborg (1193–1198) and then in the tower at Søborg Castle on Zealand until 1206. Bishop Valdemar was released upon the initiative of the Danish Queen Dagmar and Pope Innocent III and after swearing to never interfere again in Danish affairs.

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