joint ruler of Norway (1130–35), with Harald IV, whose abortive attempt (1137–39) to wrest sovereignty from Inge I Haroldsson and Sigurd II, sons of Harald IV, ended the first epoch in the period of Norwegian civil wars (1130–1240).

The son of the Norwegian king Sigurd I Magnusson, Magnus succeeded to the throne jointly with Harald IV in 1130. In 1134 war broke out between the two rulers, and the following year Magnus was seized by Harald, maimed, blinded, and put in a monastery. He was liberated in 1137 by Sigurd Slembi, who had slain Harald the year before but could not gain support for his own claim to the throne and hoped, instead, to install Magnus as king. Although Magnus won the allegiance of many chieftains who honoured him as the son of Sigurd I, his forces were defeated twice by the backers of Inge and Sigurd II. In 1139 Magnus began another attack on the forces of Inge and Sigurd II, this time from Denmark in alliance with Sigurd Slembi, but he was killed in the battle.