Magnus Haakonsson (Old Norse: Magnús Hákonarson, Norwegian: Magnus Håkonsson) was King of Norway (as Magnus VI) from 1263 to 1280 (junior king from 1257).
One of his greatest achievements was his modernisation and nationalisation of the Norwegian law-code, after which he is known as Magnus the Law-mender (Old Norse: Magnús lagabœtir, Norwegian: Magnus Lagabøte).
He was the first Norwegian monarch known to personally have used an ordinal number, although originally counting himself as "IV".
In the spring of 1280, Magnus fell ill in Bergen and died 9 May. He had already planned to have his son Eirik crowned at midsummer as co-ruler, instead Eirik now took over as sole king at the age of 12. Real power fell to a circle of advisors, prominent among them Magnus' queen Ingeborg. Magnus was remembered as a good ruler, who ruled by law rather than by the sword.