Interlingua (/ɪntərˈlɪŋɡwə/, Interlingua: [inteɾˈliŋɡwa]) is an international auxiliary language (IAL) developed between 1937 and 1951 by the American International Auxiliary Language Association (IALA). It is a constructed language of the "naturalistic" variety, whose vocabulary, grammar, and other characteristics are derived from natural languages. Interlingua literature maintains that (written) Interlingua is comprehensible to the billions of people who speak Romance languages, though it is actively spoken by only a few hundred.

Interlingua was developed to combine a simple, mostly regular grammar with a vocabulary common to a wide range of western European languages, making it easy to learn for those whose native languages were sources of Interlingua's vocabulary and grammar.

The name Interlingua comes from the Latin words inter, meaning 'between', and lingua, meaning 'tongue' or 'language'. These morphemes are the same in Interlingua; thus, Interlingua would mean 'between language'.

Interlingua focuses on common vocabulary shared by Western European languages, which are often descended from or heavily influenced by the Latin language (such as the Romance languages) and Greek language. Interlingua organizers have four "primary control languages" where, by default, a word (or variant thereof) is expected to appear in at least three of them to qualify for inclusion in Interlingua. These are English; French; Italian; and a combination of Spanish and Portuguese which are treated as a single mega-language for Interlingua purposes. Additionally, German and Russian have been dubbed "secondary control languages". While the result is often akin to Neo-Latin as the most frequent source of commonality, Interlingua words can have origins in any language, as long as they have drifted into the primary control languages as loanwords. For example, the Japanese words geisha and samurai and the Finnish word sauna are used in most Western European languages, and therefore in Interlingua as well; similarly, the Guugu Yimithirr word gangurru is used in latinized form (Interlingua: kanguru, English: kangaroo).

The maintainers of Interlingua attempt to keep the grammar simple and word formation regular, and use only a small number of roots and affixes. This is intended to make the language quicker to learn.

The American heiress Alice Vanderbilt Morris (1874–1950) became interested in linguistics and the international auxiliary language movement in the early 1920s. In 1924, Morris and her husband, Dave Hennen Morris, established the non-profit International Auxiliary Language Association (IALA) in New York City. Their aim was to place the study of IALs on a more complex and scientific basis. Morris developed the research programme of IALA in consultation with Edward Sapir, William Edward Collinson, and Otto Jespersen.

Investigations of the auxiliary language problem were in progress at the International Research Council, the American Council on Education, the American Council of Learned Societies, the British, French, Italian, and American Associations for the advancement of science, and other groups of specialists. Morris created IALA as a continuation of this work.

The IALA became a major supporter of mainstream American linguistics. Numerous studies by Sapir, Collinson, and Morris Swadesh in the 1930s and 1940s, for example, were funded by IALA. Alice Morris edited several of these studies and provided much of IALA's financial support. For example, Morris herself edited Sapir and Morris Swadesh's 1932 cross-linguistic study of ending-point phenomena, and Collinson's 1937 study of indication. IALA also received support from groups such as the Carnegie Corporation, the Ford Foundation, the Research Corporation, and the Rockefeller Foundation.