Olaus Magnus, or Magni (Magnus, i.e. Stora, great, being the family name, and not a personal epithet) (October 1490 – 1 August 1557) was a Swedish ecclesiastic and writer.

He was born at Linköping. Like his elder brother, Johannes Magnus, he obtained several ecclesiastical preferments (a canonry at Uppsala and at Linköping, and the archdeaconry of Strängnäs), and was employed on various diplomatic services (such as a mission to Rome, from Gustavus I, to procure the appointment of Johannes Magnus as archbishop of Uppsala); but on the success of the reformation in Sweden his attachment to the old church led him to accompany his brother into exile.

Settling at Rome, from 1527, he acted as his brother's secretary, and ultimately became his successor in the (now titular) archbishopric of Uppsala. Pope Paul III, in 1546, sent him to the council of Trent; later, he became canon of St Lambert in Liège; King Sigismund I of Poland also offered him a canonry at Posen; but most of his life, after his brother's death, seems to have been spent in the monastery of St Brigitta in Rome, where he subsisted on a pension assigned him by the pope.

He is best remembered as the author of the famous Historia de Gentibus Septentrionalibus (Rome, 1555), a work which long remained for the rest of Europe the chief authority on Swedish matters and is still a valuable repertory of much curious information in regard to Scandinavian customs and folk-lore.

The Historia was translated into Italian (Venice, 1565), German (Strassburg, 1567), English (London, 1658) and Dutch (Amsterdam, 1665); abridgments of the work appeared also at Antwerp (1558 and 1562), Paris (a French abridged version, 1561), Amsterdam (1586), Frankfort (1618) and Leiden (1652). Olaus also wrote a Tabula terrarum septentrionalium ... (Venice, 1539).