The Illyrian language (/ɪˈlɪriən/) was an Indo-European language or group of languages spoken by the Illyrians in Southeast Europe during antiquity. The language is unattested with the exception of personal names and placenames. Just enough information can be drawn from these to allow the conclusion that it belonged to the Indo-European language family.

In ancient sources, the term "Illyrian" is applied to a wide range of tribes settling in a large area of southeastern Europe, including Albanoi, Ardiaei, Autariatae, Dardani, Delmatae, Dassareti, Enchelei, Labeatae, Pannonii, Parthini, Taulantii and others (see list of ancient tribes in Illyria). It is not known to what extent all of these tribes formed a homogeneous linguistic group, but the study of the attested eponyms has led to the identification of a linguistic core area in the south of this zone, roughly around what is now Albania and Montenegro, where Illyrian proper is believed to have been spoken.

Little is known about the relationships between Illyrian and its neighboring languages. For lack of more information, Illyrian is typically described as occupying its own branch in the Indo-European family tree. A close relationship with Messapic, once spoken in southern Italy, has been suggested but remains unproven. Among modern languages, Albanian is often conjectured to be a surviving descendant of Illyrian, although this too remains unproven.

In the early modern era and up to the 19th century, the term "Illyrian" was also applied to the modern South Slavic language of Dalmatia, today identified as Serbo-Croatian. This language is only distantly related to ancient Illyrian, as they share the common ancestor Proto-Indo-European; the two languages were never in contact as Illyrian had become extinct before the Slavic migrations to the Balkans with the possible exception of the ancestor of Albanian.

Illyrian proper went extinct between the 2nd and 6th centuries AD, with the possible exception of a branch that may have survived and developed into Albanian.

It has also been claimed that Illyrian was preserved and spoken in the countryside, as attested in the 4th-5th century testimonies of St. Jerome.

Illyrian was part of the Indo-European language family. Its relation to other Indo-European languages, ancient and modern, is poorly understood because of the paucity of data and because it is still being examined. Today, the main source of authoritative information about Illyrian consists of a handful of Illyrian words cited in classical sources and numerous examples of Illyrian anthroponyms, ethnonyms, toponyms and hydronyms. The scarcity of the data makes it difficult to identify the sound changes that have taken place in Illyrian; the most widely accepted one is that the Indo-European voiced aspirates /bʰ/, /dʰ/, /ɡʰ/ became voiced consonants /b/, /d/, /ɡ/.

Messapic was an Iron Age language spoken in Apulia by the Iapygians (Messapians, Peucetians, Daunians), which settled in Italy as part of an Illyrian migration from Illyria in the transitional period between the Bronze and Iron ages. As such, Messapic, as a distinct language, is considered to be part of the same Paleo-Balkan grouping as Illyrian. Eric Hamp has grouped them under "Messapo-Illyrian" which is further grouped with Albanian under "Adriatic Indo-European". Other schemes group the three languages under "General Illyrian" and "Western Paleo-Balkan".