The Kalmar Union (Danish, Norwegian, and Swedish: Kalmarunionen; Finnish: Kalmarin unioni; Latin: Unio Calmariensis) was a personal union in Scandinavia, agreed at Kalmar in Sweden, that from 1397 to 1523 joined under a single monarch the three kingdoms of Denmark, Sweden (then including most of present-day Finland), and Norway, together with Norway's overseas colonies (then including Iceland, Greenland, the Faroe Islands, and the Northern Isles of Orkney and Shetland).

The union was not quite continuous; there were several short interruptions. Legally, the countries remained separate sovereign states. However, their domestic and foreign policies were directed by a common monarch. Gustav Vasa's election as King of Sweden on 6 June 1523, and his triumphant entry into Stockholm eleven days later, marked Sweden's final secession from the Kalmar Union. Formally, the Danish king acknowledged Sweden's independence in 1524 at the Treaty of Malm.

Gustav I, born Gustav Eriksson of the Vasa noble family and later known as Gustav Vasa (12 May 1496 – 29 September 1560), was King of Sweden from 1523 until his death in 1560, previously self-recognised Protector of the Realm (Riksföreståndare) from 1521, during the ongoing Swedish War of Liberation against King Christian II of Denmark, Norway and Sweden. Gustav rose to lead the rebel movement following the Stockholm Bloodbath, where his father was executed. Gustav's election as king on 6 June 1523 and his triumphant entry into Stockholm eleven days later marked Sweden's final secession from the Kalmar Union.As king, Gustav proved an energetic administrator with a ruthless streak not inferior to his predecessor's, brutally suppressing subsequent uprisings (three in Dalarna – which had once been the first region to support his claim to the throne – one in Västergötland, and one in Småland). He worked to raise taxes and bring about a Reformation in Sweden, replacing the prerogatives of local landowners, noblemen and clergy with centrally appointed governors and bishops. His 37-year rule, which was the longest of a mature Swedish king to that date (subsequently passed by Gustav V and Carl XVI Gustav) saw a complete break with not only the Danish - Norwegian supremacy but also the Roman Catholic Church, whose assets were nationalised, with the Lutheran Church of Sweden established under his personal control. He became the first truly autocratic native Swedish sovereign and was a skilled bureaucrat and propagandist, with tales of his largely fictitious adventures during the liberation struggle still widespread to this day. In 1544, he abolished Medieval Sweden's elective monarchy and replaced it with a hereditary monarchy under the House of Vasa, which held the Swedish throne until 1654. Thus, three of his sons, Eric XIV, John III and Charles IX, all held the kingship at different points.Gustav I has subsequently been labelled the founder of modern Sweden, and the "father of the nation". Gustav liked to compare himself to Moses, whom he believed to have also liberated his people and established a sovereign state. As a person, Gustav was known for ruthless methods and a bad temper, but also a fondness for music and had a certain sly wit and ability to outmaneuver and annihilate his opponents. He founded one of the now oldest orchestras of the world, the Kungliga Hovkapellet (Royal Court Orchestra). Royal housekeeping accounts from 1526 mention twelve musicians including wind players and a timpanist but no string players. Today the Kungliga Hovkapellet is the orchestra of the Royal Swedish Opera.