Árni Magnússon (13 November 1663 – 7 January 1730) was an Icelandic-Danish scholar and collector of manuscripts. He assembled the Arnamagnæan Manuscript Collection.
Árni was born in 1663 at Kvennabrekka in Dalasýsla, in western Iceland, where his father Magnús Jónsson was the minister (and later prosecutor and sheriff). His mother was Guðrún Ketilsdóttir, daughter of archdeacon Ketill Jörundarson of Hvammur. He was raised by his grandparents and uncle. At 17 he entered the Cathedral School in Skálholt, then three years later, in 1683, went to Denmark (with his father, who was part of a trade lobbying contingent) to study at the University of Copenhagen. There he earned the degree of attestus theologiæ after two years, and also became an assistant to the Royal Antiquarian, Thomas Bartholin, helping him prepare his Antiquitates danicæ and transcribing, translating, and annotating thousands of pages of Icelandic material.
After Bartholin's death in 1690, Árni became librarian and secretary to a Danish statesman, Matthias Moth, brother of the king's mistress, Sophie Amalie Moth. From late 1694 to late 1696, he was in Germany, primarily to assess a book collection that had been offered to the university, but he extended the stay. Meanwhile, presumably thanks to his employer, he was appointed a professor designate at the university, and since he had never published, while still in Leipzig he published an edition of some Danish chronicles that he had copied while working for Bartholin. When he returned to Denmark, he resumed working for Moth but in 1697 was also appointed secretary at The Royal Secret Archives (Det Kongelige Gehejmearkiv). In 1699 he published anonymously, at Moth's request, an account of a witchcraft case in which Moth had been a judge, Kort og sandfærdig Beretning om den vidtudraabte Besættelse udi Thisted.
With vice-lawman Páll Vídalín, he was assigned by the king to survey conditions in Iceland; this took ten years, from 1702 to 1712, most of which time he spent there. He returned to Copenhagen for two winters, the first time to present trade proposals, and during the second, which was in connection with a court case, he got married. The ultimate results of the survey were the Icelandic census of 1703 and the Jarðabók or land register, surveying for which was not completed until 1714 and which had to be translated into Danish after Árni's death. He was expected to translate it himself, but it was one of several official tasks he neglected. It was finally published in 11 volumes in 1911–41. However, the mission as set out by the king was extremely broad, including investigating the feasibility of sulphur mining, assessing the fisheries, and auditing the administration of justice. Complaints of judicial abuse poured in, and officials were incensed by the two men's inquiries into past court cases and in turn complained to Copenhagen about them.