Margrethe I (1353-1412) was Queen Regent of Denmark, Norway and Sweden and one of the most powerful women in medieval Europe. She was the youngest child of King Valdemar IV Atterdag and Queen Helvig of Schleswig. During her reign, Margrethe founded the Kalmar Union which united all of Scandinavia under one crown.

Margrethe was married, at the age of ten, to King Haakon VI of Norway and Sweden in 1363 and was taken to Norway. Margrethe supported the Birgittine Order her whole life and was instrumental in the establishment of the order in Denmark.

In 1364, Haakon was overthrown as King of Sweden by Albert of Mecklenburg but maintained the crown of Norway. When king Valdemar died in 1375, Margrethe's and Haakon's only son, Olaf II, became king of Denmark, with Margrethe as Queen Regent. In 1380, king Haakon past away and so, Olaf inherited the kingdom of Norway. However, Margrethe also functioned as Queen Regent on behalf of Olaf in Norway. In 1387, Olaf died leaving Margrethe as sole power in Denmark and Norway.

In 1389, the Swedish nobles had grown tired of the misrule of Albert of Mecklenburg and invited Margrethe to overthrow him. After a series of campaigns, Margrethe defeated Albert in battle and became Queen Regent of Sweden. In 1397 she summoned a congress of Councils of the Realm at Kalmar to form a union. On June 17th, her great-nephew Eric of Pomerania was crowned King of Denmark, Norway and Sweden. The Kalmar Union united all of Scandinavia under one crown and lasted for over a century.

Margrethe was effectively Queen Regent for the rest of her life and the real ruler of the Kalmar Union. To further extend her power, she arranged for an alliance with Henry IV of England. In 1406, his daughter Philippa married Eric of Pomerania.

Margrethe died of the plague in 1412 and is buried in a magnificent tomb at Roskilde Cathedral.