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

 He was the founder of the so-called Kazan, later the St. Petersburg, linguistic school. He was a professor at the universities of Kazan (1875–83), Iur’ev (today Tartu; 1883–93), Kraków (1893–99), and St. Petersburg (1900– 18).

 In , he was a professor at the re- established Warsaw University in a once again independent Poland. Baudouin de Courtenay's view of language as structural entities anticipated later interest in the connection between structure and meaning, as well as directly influencing the structuralist theory of Ferdinand de Saussure. He was also fascinated by the relationship between language and nationality, advocating the peaceful co-existence and cooperation of all ethnic groups and nationalities, without the dominance or cultural assimilation of any group by another.

 Throughout his life, Baudouin de Courtenay published hundreds of scientific works in Polish, Russian, Czech, Slovenian, Italian, French, and German.  His work had a major impact on twentieth century linguistic theory, and it served as a foundation for several schools of phonology. Together with his student, Mikołaj Kruszewski, de Courtenay coined the term phoneme.  He was an early champion of synchronic linguistics, the study of contemporary spoken languages, and he had a strong impact on the structuralist linguistic theory of Swiss linguist Ferdinand de Saussure, among whose notable achievements is the distinction between statics and dynamics of languages and between a language, that is an abstract group of elements) and speech (its implementation by individuals).

 The origin and development of modern quantitative linguistics is associated with the structuralist revolution of the first decades of the twentieth century, and particularly with the work of Baudouin de Courtenay. While he did not apply mathematical methods himself, he did, while conducting field studies, realize the virtues of a quantitative description of language. He foresaw the advent of rigorous investigations into the laws of language, and articulated them in his 1927 Quantity as a Dimension of Thought about Language.  Baudouin de Courtenay's concept principally involved the semantic, syntactic, and morphologic representations of the number, dimensions, and intensities of the attributes. Thus he did not touch upon the concept of statistical linguistics operating with frequencies or other expressly numerical features of the language elements. Nonetheless, he perceived analogies between the physical domain, defined by precise and formalized laws, and language.

 Jan Baudouin de Courtenay made a lasting contribution to phonology and foreshadowed the development of mathematical linguistics. He pioneered the scientific approach to contrastive and applied linguistics, inspired new theoretical and cognitive trends in lexicology, semantics, onomastics and anthroponymy, as well as in dialectology, sociolinguistics, and logopedics.