Finnish nationalist and pioneer in the study of remote Arctic and Siberian Uralic and Altaic languages. He also championed the ideology of Pan-Turanianism—the belief in the racial unity and future greatness of the Ural-Altaic peoples.

After many years of field research in Siberia, Castrén made important contributions to the study of the lesser-known Uralic, Altaic, and Paleo-Siberian languages. He further concluded that the Finns originated in Central Asia and that, far from being a small, isolated people, they were part of a larger polity that included such groups as the Magyars, the Turks, and the Mongols. This belief was accepted by the Finnish nationalists after Castrén, himself a zealous nationalist, made his views public in 1849 and lent great impetus to the advancement of Finnish language study in Finland. Castrén occupied the first chair in Finnish at the University of Helsinki (1851) and became university chancellor the following year. His most significant and lasting contribution is his detailed analysis of individual Samoyedic languages, which provided the first sound comparative basis for uniting the Finno-Ugric and Samoyedic languages into a common Uralic family.