Magnus Haraldsson (Old Norse: Magnús Haraldsson) was King of Norway from 1066 to 1069, jointly with his brother Olaf Kyrre from 1067. He was not included in official Norwegian regnal lists until modern times, but has since been counted as Magnus II.
A son of King Harald Hardrada, Magnus was in 1058 appointed nominal leader of an expedition into the Irish Sea while still only a child. He appears to have assisted Welsh ruler Gruffydd ap Llywelyn and Ælfgar, Earl of Mercia in their struggles against the English, although his primary objective may have been to assert control over Orkney. He later accompanied his father in Harald's campaign against Denmark in 1062, and was appointed regent and made king before Harald's fatal invasion of England in 1066. Magnus briefly ruled Norway alone thereafter, until his younger brother Olaf returned from England in 1067.
Magnus co-ruled with Olaf following his brother's return to Norway, but less than three years into his reign, Magnus became ill and died. His kingship has been downplayed in later history in part due to this. Magnus had only one child, Haakon Toresfostre who was king briefly after Olaf, but who also died young, and without issue.