Magnus Haakonsson (1 May 1238 – 9 May 1280), known as Magnus the Lawmender, is the King of Norway from 1263 to 1280.

His greatest achievement is his modernization and nationaliation of the Norwegian law-code, which is what gives him his epithet.

He is the first Norwegian monarch who is known to have used an ordinal number, although he counts himself as IV instead of the modern VI.