German linguist and Celtic scholar who was one of the first to use the principles of modern historical linguistics in the field of Celtic studies. He was also an excellent Latinist.

Thurneysen taught at the universities of Jena (1885–87), Freiburg (1887–1913), and Bonn (1913 until the end of his academic career). He is known for a wide range of contributions to the study of Old Irish linguistic, literary, and legal history; and indeed he is usually considered to have been the greatest scholar ever in these fields. His best known work is Handbuch des Altirischen, 2 vol. (1909; A Grammar of Old Irish). Other major works include Die irische Helden- und Königssage bis zum 17.ten Jahrhundert (1921; “The Irish Sagas of Heroes and Kings Until the 17th Century”) and Das Keltische Recht (1935; “Celtic Law”).