Karl (Kai) Reinhold Donner (April 1, 1888 in Helsinki – February 12, 1935) was a Finnish linguist, ethnographer and politician. He carried out expeditions to the Nenets people (Samoyeds) in Siberia 1911–1914 and was docent of Uralic languages at the University of Helsinki from 1924. He was, among other things, a pioneer of modern anthropological fieldwork methods, though his work is little known in the English-speaking world. He was the father of the Finnish politician and film producer Jörn Donner and geologist Joakim Donner. He is buried in the Hietaniemi Cemetery in Helsinki.