Valdemar, Duke of Finland

Valdemar Magnusson (1280 – 1319) was a Swedish prince, heir to the throne of Sweden, and Duke of Finland.

Valdemar was the third son of Magnus III and Helvig of Holstein. He became Duke of Finland in 1302 at the coronation of his older brother, Birger. Valdemar married Kristina, daughter of Torgils Knutsson, who was the constable and virtual ruler of Sweden during King Birger's early reign.

Valdemar's father-in-law was arrested in December 1305 and executed the following February. Valdemar subsequently divorced his wife, claiming that they were spiritually related (baptism siblings) because Torgils was his godfather. In the fall of 1312 in Oslo, he married Ingeborg, daughter of King Eirik II of Norway. It was a double wedding, because at the same time Valdemar's brother Eric married Ingeborg, daughter of King Haakon V of Norway. Valdemar and Ingeborg had a son in 1316 who died as a child.

In 1306, in an event known as the Håtuna games (Håtunaleken), King Birger was captured by his brothers on the Håtuna royal estate in Uppland and taken as prisoner to Nyköping Castle (Nyköpingshus). Two years later, King Eric VI of Denmark forced Valdemar and Eric to release King Birger under humiliating conditions. King Birger sought aid in Denmark after his release and the strife resumed.

By 1315, Valdemar and Eric had managed to wrest large parts of the Swedish kingdom from their brother. Valdemar had gained Turku and Häme castles, with a lion's share of Finland, the castle of , most of Uppland, Borgholm castle, and the island of Öland. Eric was in possession of Kungahälla (which he had been given during his exile by the Norwegian king) and northern Halland, which he had been given by King Eric VI of Denmark.

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