Matthias Alexander Castrén (December 2, 1813-May 7, 1853) was a Finnish ethnologist and philologist.

Castren was born at Tervola, in the parish of Kemi in Finland, on the 20th of November (December 2, 1813). His father, Christian Castrén, parish minister at Rovaniemi, died in 1825; and Matthias passed under the protection of his uncle, Mathias Castrén, the kindly and learned incumbent of Kemi. At the age of twelve he was sent to school at Uleåborg, and there he helped to maintain himself by teaching the younger children. On his removal to the Alexander University at Helsingfors in 1828 he first devoted himself to Greek and Hebrew with the intention of entering the church; but his interest was soon excited by the language of his native country, and he even began before his course was completed to lay the foundations of a work on Finnish mythology.

The necessity of personal explorations among the still unwritten languages of cognate tribes soon made itself evident; and in 1838 he joined a medical fellowstudent, Dr. Ehrstrom, in a journey through Lapland. In the following year he travelled in Russian Karelia at the expense of the Literary Society of Finland; and in 1841 he undertook, in company with Dr Elias Lönnrot, the great Finnish philologist, a third journey, which ultimately extended beyond the Ural as far as Obdorsk, and occupied a period of three years. Before starting on this last expedition he had published a translation into Swedish of the Finnish epic of Kalevala; and on his return he gave to the world his Elementa grammatices Syrjaenae and Elementa grammatices Tscheremissae, 1844.

No sooner had he recovered from the illness which his last journey had occasioned than he set out, under the auspices of the Academy of St Petersburg and the Helsingfors University, on an exploration of the whole government of Siberia, which resulted in a vast addition to previous knowledge, but seriously affected the health of the adventurous investigator. The first-fruits of his collections were published at St Petersburg in 1849 in the form of a Versuch einer ostjatischen Sprachlehre. In 1850 he published a treatise De affixis personalibus linguarum Altaicarum, and was appointed professor at Helsingfors of the new chair of Finnish language and literature. The following year saw him raised to the rank of chancellor of the university; and he was busily engaged in what he regarded as his principal work, a Samoyedic grammar, when he died on the 7th of May 1853.

Five volumes of his collected works appeared from 1852 to 1858, containing respectively (1) Reseminnen från åren 1838-1844; (2) Reseberättelser och bref åren 1845-1849; (3) Föreläsningar i finsk mytologi; (4) Ethnologiska föreläsningar öfver altaiska folken; and (5) Smärre afhandlingar och akademiska dissertationer. A German translation was published by Anton Schiefner, who was also entrusted by the St Petersburg Academy with the editing of his manuscripts, which had been left to the Helsingfors University and which were subsequently published.