Jomsborg or Jómsborg was a semi-legendary Viking stronghold at the southern coast of the Baltic Sea, that existed between the 960s and 1043. Its inhabitants were known as Jomsvikings. Jomsborg's exact location, or its existence, has not yet been established, though it is often maintained that Jomsborg was located on the eastern outlet of the Oder river. Historian Lauritz Weibull dismissed Jomsborg as a legend.

Vikings were seafaring people originally from Scandinavia, who from the late 8th to the late 11th centuries raided, pirated, traded, and settled throughout parts of Europe. They also voyaged as far as the Mediterranean, North Africa, the Middle East, Greenland, and Vinland. In their countries of origin, and some of the countries they raided and settled in, this period is popularly known as the Viking Age, and the term "Viking" also commonly includes the inhabitants of the Scandinavian homelands as a whole. The Vikings had a profound impact on the early medieval history of Scandinavia, the British Isles, France, Estonia, and Kievan Rus'.