Karl (Kai) Reinhold Donner (1 April 1888 in Helsinki – 12 February 1935) was a Finnish linguist, ethnographer and politician.

He carried out expeditions to the Ob-Ugric and Samoyedic peoples[1] in Siberia 1911–1914 and was docent of Uralic languages at the University of Helsinki from 1924.

Kai Donner had decided early on that he wanted to follow in the footsteps of pioneer philologist and explorer M.A.

Living with the Nenets and Khanty people, Donner studied not only the language but also the way of life and beliefs of his hosts.

Finland-Swedish by mother tongue, he expressed reservations about the persecution of Swedish speakers, which was commonly supported by conservative Finns in those decades.