Johann Kaspar Zeuss (or Zeuß, 22 July 1806 – 10 November 1856) was a German historian and founder of Celtic philology.

In 1837 his book Die Herkunft der Baiern von den Markomannen (The Descent of the Bavarians from the Markomans), brought him an honorary PhD from the University of Erlangen.

His Germanic studies had taught him the necessity of knowing Celtic languages, and he went to work to investigate this neglected field.

To get at the sources, the old manuscripts, particularly those in Old Irish, he journeyed to Karlsruhe, Würzburg, St. Gallen, Milan, London, and Oxford, and everywhere made extracts or copies.

De Barra (2018, page 26) writes that Zeuss "brought rigorous scientific methodologies and a sense of scholarly gravitas to the study of indigenous languages of Britain and Ireland....something the field had lacked before....in his wake universities in Britain and Europe devoted more resources to the study of Celtic matters".