André Martinet holds an important position in the history of linguistics in the twentieth century. For more than six decades he decisively influenced the development of linguistics in France and in the world. He is one of the spokespersons for French linguistic structuralism, the structuralisme fonctionnel. The article focuses on a description and critical appreciation of the interlinguistic part of Martinet’s work. The issue of auxiliary languages and hence interlinguistics had interested Martinet greatly from his youth and provoked him to examine the matter actively. From 1946 onwards he worked in New York as a professor at Columbia University and a research director of the International Auxiliary Language Association (IALA). From 1934 he was in contact with the Danish linguist and interlinguist Otto Jespersen (1860–1943). Martinet, who went back to Paris in 1955 to work as a professor at the École Pratique des Hautes Études (Sorbonne), increasingly developed into an expert in planned languages; for his whole life, he was committed to the world-wide use of a foreign language that can be learned equally easily by members of all ethnic groups; Esperanto, functioning since 1887, seemed a good option to him.