Jan Niecisław Ignacy Baudouin de Courtenay (13 March 1845 – 3 November 1929) was a Polish linguist and Slavist, best known for his theory of the phoneme and phonetic alternations.

For most of his life Baudouin de Courtenay worked at Imperial Russian universities: Kazan (1874–1883), Dorpat (as Tartu, Estonia was then known) (1883–1893), Kraków (1893–1899) in Austria-Hungary, and St. Petersburg (1900–1918), where he was known as Иван Александрович Бодуэн де Куртенэ (Ivan Aleksandrovich Boduen de Kurtene), and in Russia he is recognized as a Russian scientist. In 1919-1929 he was a professor at the re-established University of Warsaw in a once again independent Poland.