Theodor "Thore" Magnus Fries (28 October 1832 – 29 March 1913), was a Swedish botanist, lichenologist, and Arctic explorer. He was the son of the mycologist Elias Fries. Following in his father's footsteps, Fries studied botany, obtaining his doctoral degree in 1857 at Uppsala. He became a member of the Royal Swedish Academy of Sciences in 1865 and professor of botany and applied economics at Uppsala in 1877. His most notable work was Lichenographia scandinavica (1871–1874). He also produced a two-volume biography of Carl Linnaeus (1903).