king of Norway (1355–80) whose marriage to Margaret, daughter of the Danish king Valdemar IV, in 1363 paved the way for the eventual union (1397) of the three major Scandinavian nations—Denmark, Norway, and Sweden—the Kalmar Union. Haakon was deeply embroiled throughout his reign in political conflicts with Sweden, Denmark, and the cities of the north German trading confederation, the Hanseatic League.

The younger son of Magnus VII (Magnus II Eriksson) Eriksson, king of Norway and Sweden, Haakon was named his father's successor in Norway in 1343 and became king there in 1355, five years after the nation had been devastated by the Black Death, probably bubonic plague. The plague had killed large numbers of the nobility, clergy, and civil servants, weakening the power of both the aristocracy and the royal administration. The Swedish nobility remained strong, however, and, under the leadership of Haakon's brother Erik, rebelled against the rule of Magnus VII. Haakon came to his father's aid and was named joint king of Sweden in 1362 after Erik's death.

Haakon again assisted Magnus against the rebellious Swedish nobles in 1364, but the two kings were defeated, and Haakon retreated while his father was taken prisoner. A temporary agreement (1370) with the leaders of the Hanseatic League, who had launched a war against Norway and Denmark in 1367, freed him to rescue his father in 1371. He conceded special trading privileges to the Hanseatic merchants in a final peace treaty (1376), which helped secure the right to the Danish throne for his son Olaf V (1370–87) by placating Danish magnates fearful of Hanseatic intervention. Olaf also succeeded to the Norwegian throne on Haakon's death (1380), but he died in 1387 at the age of 17, leaving his mother (Haakon's widow), Margaret, to rule in both Denmark and Norway.