Magnus Haakonsson (Old Norse: Magnús Hákonarson, Norwegian: Magnus Håkonsson; 1 May 1238 – 9 May 1280) 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".

He was the youngest son of King Håkon Håkonsson and his wife Margaret Skuladotter. He was born in Tunsberg and was baptised in May 1238. He spent most of his upbringing in Bergen. In 1257 his older brother Håkon died, leaving Magnus the heir-apparent to the kingdom. His father gave him the title of king the same year. On 11 September 1261, he married the Danish princess Ingeborg, the daughter of the late Danish King Erik Plogpenning, after she was practically abducted by King Håkon's men from the monastery she was living in. The struggle to claim Ingeborg's inheritance from her murdered father later involved Norway in intermittent conflicts with Denmark for decades to come. Magnus and Ingeborg were crowned directly after their marriage, and Magnus was given Ryfylke for his personal upkeep. On 16 December 1263 King Håkon died while fighting the Scottish king over the Hebrides, and Magnus became the ruler of Norway.

Magnus' rule brought about a change from the somewhat aggressive foreign policy of his father. In 1266 he gave up the Hebrides and the Isle of Man to Scotland, in return for a large sum of silver and a yearly payment, under the Treaty of Perth, by which the Scots at the same time recognised Norwegian rule over Shetland and the Orkney Islands. In 1269 the Treaty of Winchester cemented good relations with the English king Henry III. Magnus also seems to have had good relations with the Swedish King Valdemar Birgersson, and in the 1260s, the border with Sweden was officially defined for the first time. When Valdemar was deposed by his two brothers and fled to Norway in 1275, this stirred Magnus into gathering a leidang-fleet for the first and only time in his reign. With a large fleet, he met with the new Swedish King Magnus Ladulås to try to bring about a settlement between the two brothers, but without success, Magnus of Sweden would not give in to pressure and the Magnus of Norway retreated without engaging in hostile actions.

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