Jan Niecisław Ignacy Baudouin de Courtenay, also Ivan Alexandrovich Baudouin de Courtenay (Russian: Иван Александрович Бодуэн де Куртенэ; 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 (now Estonia) (1883–1893), Kraków (1893–1899) in Austria-Hungary, and St. Petersburg (1900–1918). In 1919–1929 he was a professor at the re-established University of Warsaw in an again independent Poland.

He was born in Radzymin, in the Warsaw Governorate of Congress Poland (a state in personal union with the Russian Empire), to a family of distant French extraction. One of his ancestors had been a French aristocrat who immigrated to Poland during the reign of Polish King Augustus II the Strong.

In 1862 Baudouin de Courtenay entered the "Main School," a predecessor of the University of Warsaw. In 1866 he graduated from its historical and philological faculty and won a scholarship of the Russian Imperial Ministry of Education. After leaving Poland, he studied at various foreign universities, including those of Prague, Jena and Berlin. In 1870 he received a doctorate from the University of Leipzig for his work on analogy and a master's degree from St. Petersburg for his Polish-language dissertation On the Old Polish Language Prior to the 14th Century.

Baudouin de Courtenay established the Kazan School of linguistics in the mid-1870s and served as professor at the local university from 1875. Later he was chosen as the head of linguistics faculty at the University of Dorpat (1883–1893). In 1882 he married historian and journalist Romualda Bagnicka. Between 1894 and 1898 he occupied the same post at the Jagiellonian University in Kraków only to be appointed to St. Petersburg, where he continued to refine his theory of phonetic alternations. After Poland regained independence in 1918, he returned to Warsaw, where he formed the core of the linguistics faculty of the University of Warsaw. From 1887 he held a permanent seat in the Polish Academy of Skills and from 1897 he was a member of the Petersburg Academy of Sciences.

Baudouin de Courtenay was the editor of the 3rd (1903–1909) and 4th (1912–1914) editions of the Explanatory Dictionary of the Living Great Russian Language compiled by Russian lexicographer Vladimir Dahl (1801–1872).

Apart from his scientific work, Baudouin de Courtenay was also a strong supporter of the national revival of various national minority and ethnic groups. In 1915 he was arrested by the Okhrana, the Russian secret service, for publishing a brochure on the autonomy of peoples under Russian rule. He spent three months in prison, but was released. In 1922, without his knowledge, he was proposed by the national minorities of Poland as a presidential candidate, but was defeated in the third round of voting in the Polish parliament and eventually Gabriel Narutowicz was chosen. He was also an active Esperantist and president of the Polish Esperanto Association. In 1925, he was one of the co-founders of the Polish Linguistic Society.

In 1927, he formally withdrew from the Catholic Church without joining any other religious denomination. He died in Warsaw. He is buried at the Protestant Reformed Cemetery in Warsaw with the epitaph "He sought truth and justice".