Valdemar II (9 May 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 of Denmark and Sophia of Minsk, the daughter of Richeza of Poland, Queen Dowager of Sweden and Volodar Giebovich, Prince of Minsk. When his father died, young Valdemar was only twelve years old. He was named Duke of Southern Jutland (Latin: dux slesvicensis, literally Duchy of Schleswig duke), represented by the regent Bishop Valdemar Knudsen, the illegitimate son of King Canute V of Denmark.

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

Duke Valdemar realized the threat Bishop Valdemar represented. He thus invited the Archbishop to meet him in Aabenraa in 1192. The Bishop then 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 based on the fact that he was the son of King Canute V. In 1193, King Canute VI captured him. Bishop Valdemar stayed in captivity in Nordborg (1193–1198) and then in the tower at Søborg Castle on Zealand until 1206. He was later released upon the initiative of Dagmar of Bohemia (the wife of Duke Valdemar) and Pope Innocent III, after swearing to never interfere again in Danish affairs.

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