Interlingua (; ISO 639 language codes ia, ina) is an international auxiliary language (IAL) developed between 1937 and 1951 by the American International Auxiliary Language Association (IALA). It ranks among the most widely used IALs and is the most widely used naturalistic IAL – in other words, those IALs whose vocabulary, grammar, and other characteristics are derived from natural languages, rather than being centrally planned. Interlingua literature maintains that (written) Interlingua is comprehensible to the hundreds of millions 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 the widest possible range of western European languages, making it unusually easy to learn, at least for those whose native languages were sources of Interlingua's vocabulary and grammar. Conversely, it is used as a rapid introduction to many natural languages.

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'.