Magnus I (Swedish: Magnus Nilsson ; Danish: Magnus Nielsen) (born about 1106, died 4 June 1134 in the Battle of Fotevik), was a Danish duke who ruled Gothenland in southern Sweden from the 1120s to c. 1132. His status as ruler of Sweden was disputed in his own time, but today he is recognized as one of the historical Swedish monarchs.Snorri Sturlason gives him the epithet Magnus the Strong.

Magnus was the son of King Niels I of Denmark and Margaret Fredkulla, the second or eldest daughter of King Inge the Elder of Sweden. His elder brother Inge was killed in a riding accident, leaving Magnus as the sole heir to Niels. He grew up to be a tall and strong young man, a head taller than anyone else. The chronicles give different opinions about his character, depending on their political preferences. The near-contemporary Roskilde Chronicle calls him merry and generous and a "lover of firmness in character". Saxo Grammaticus, on the other hand, says that he was well endowed by nature but still a violent brute.

When Margaret's first cousin King Inge the Younger died at an unknown time in the 1120s, Magnus claimed the throne as the eldest grandson of Inge the Elder. According to the chronicler Saxo Grammaticus, Magnus was recognized by the Geats (Götarna) of Gothenland, but according to the Westrogothic law, the prerogatives of selecting a king lay with the Svear, another tribe to the north of the Geats. The Svear, on their part, had selected Ragnvald Knaphövde, whose saga was however short. According to the brief chronicle incorporated in the Westrogothic law, Ragnvald showed disrespect towards the Geats by not giving hostages when riding his tour of installation. In retaliation, Ragnvald was murdered by the Geatish population, an event sometimes dated to c. 1129. "At his death", according to Saxo, "power was transferred to Magnus". Magnus is not mentioned as King in the Westrogothic law, nor in any Swedish king-list, leaving a question-mark around his actual sphere of power. The law-speaker of Västergötland, Karl of Edsvära, governed his province around this time and is occasionally known in the sources as jarl or even "king".

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