The Kalmar Union or Union of Kalmaris (Danish, Norwegian and Swedish: Kalmarunionen; Latin: Unio Calmariensis) is a state that brings together the Scandinavian nations from 1397 to 1523.

It is a personal union that joins under a single monarch the three kingdoms of Denmark, Sweden (at this time including Finland), and Norway, together with Norway's overseas dependencies (including Iceland, Greenland, the Faroe Islands and the Northern Isles).

The Union is not quite continuous; there are several short interruptions.

Legally the countries remain separate sovereign states, but with their domestic and foreign policies being directed by a common monarch.One main impetus for its formation is to block German expansion northward into the Baltic region.

The main reason for its failure to survive is the perpetual struggle between the monarch, who wants a strong unified state, and the Swedish and Danish nobility which does not.

Diverging interests (especially the Swedish nobility's dissatisfaction with the dominant role played by Denmark and Holstein) give rise to a conflict that hamper the union in several intervals from the 1430s until its definitive breakup in 1523 when Gustav Vasa becomes king of Sweden.

Norway will continue to remain a part of the realm of Denmark–Norway under the Oldenburg dynasty for nearly three centuries until its dissolution in 1814.In 1397, the land area of the union is as large as three million square kilometers under the rule of Margaret I of Denmark.