Swedish pre-history ends around 800 AD, when the Viking Age begins and written sources are available. The Viking Age lasted until the mid-11th century. Scandinavia was formally Christianized by 1100 AD. The period 1050 to 1350—when the Black Death struck Europe—is considered the Older Middle Ages. The Kalmar Union between the Scandinavian countries was established in 1397 and lasted until King Gustav Vasa ended it upon seizing power. The period 1350 to 1523 – when king Gustav Vasa, who led the unification of Sweden in the Swedish War of Liberation, was crowned – is considered the Younger Middle Ages. During these centuries, Sweden gradually consolidated as a single nation.

The Kalmar Union was a personal union in Scandinavia, agreed at Kalmar in Sweden as designed by widowed Queen Margaret of Norway and Sweden. From 1397 to 1523, it joined under a single monarch the three kingdoms of Denmark, Sweden, and Norway, together with Norway's overseas colonies.