Kazimieras Jaunius (1848 - 1908) was a Lithuanian priest and linguist. He started his studies in linguistics in the end of 19th century in Kaunas. While Jaunius did not publish much, his major achievements include research on the similarity of Lithuanian to other languages and the systematization of research of Lithuanian dialects. He collaborated with Jan Niecisław Baudouin de Courtenay and Jonas Basanavičius.
His famous disciple is Kazimieras Būga, who from 1903 worked as Jaunius' personal secretary. Būga collected most of Jaunius' work, who was known not to be keen to write his thoughts down. Būga prepared four volumes of Jaunius' writings for print.