The Illyrians (Ancient Greek: Ἰλλυριοί, Illyrioi; Latin: Illyrii) were a group of Indo-European-speaking people who inhabited the western Balkan Peninsula in ancient times. They constituted one of the three main Paleo-Balkan populations, along with the Thracians and Greeks.

The territory the Illyrians inhabited came to be known as Illyria to later Greek and Roman authors, who identified a territory that corresponds to most of Albania, Montenegro, Kosovo, much of Croatia and Bosnia and Herzegovina, western and central Serbia and some parts of Slovenia between the Adriatic Sea in the west, the Drava river in the north, the Morava river in the east and the Ceraunian Mountains in the south. The first account of Illyrian people dates back to the 6th century BC, in the works of the ancient Greek writer Hecataeus of Miletus.

The name "Illyrians", as applied by the ancient Greeks to their northern neighbors, may have referred to a broad, ill-defined group of people. It has been suggested that the Illyrian tribes never collectively identified as "Illyrians", and that it is unlikely that they used any collective nomenclature at all. Illyrians seems to be the name of a specific Illyrian tribe who were among the first to encounter the ancient Greeks during the Bronze Age. The Greeks later applied this term Illyrians, pars pro toto, to all people with similar language and customs.

In archaeological, historical and linguistic studies, research about the Illyrians, from the late 19th to the 21st century, has moved from Pan-Illyrian theories, which identified as Illyrian even groups north of the Balkans to more well-defined groupings based on Illyrian onomastics and material anthropology since the 1960s as newer inscriptions were found and sites excavated. There are two principal Illyrian onomastic areas: the southern and the Dalmatian-Pannonian, with the area of the Dardani as a region of overlapping between the two. A third area, to the north of them – which in ancient literature was usually identified as part of Illyria – has been connected more to the Venetic language than to Illyrian. Illyric settlement in Italy was and still is attributed to a few ancient tribes which are thought to have migrated along the Adriatic shorelines to the Italian peninsula from the geographic "Illyria": the Dauni, the Peuceti and Messapi (collectively known as Iapyges, and speaking the Messapic language).

The term "Illyrians" last appears in the historical record in the 7th century, referring to a Byzantine garrison operating within the former Roman province of Illyricum. What happened to the Illyrians after the settlement of the Slavs in the region is a matter of debate among scholars, and includes the hypothesis of the origin of the Albanian language from the Illyrian language, which is often supported by scholars for obvious geographic and historical reasons but not proven.

While the Illyrians are largely recorded under the ethnonyms of Illyrioi (Ἰλλυριοί) and Illyrii, these appear to be misspelt renditions by Greek or Latin-speaking writers. Based on historically attested forms denoting specific Illyrian tribes or the Illyrians as a whole (e.g., Úlloí (Ύλλοί) and Hil(l)uri), the native tribal name from which these renditions were based has been reconstructed by linguists such as Heiner Eichner as *Hillurio- (< older *Hullurio-). According to Eichner, this ethnonym, translating to 'water snake', is derived from Proto-Indo-European *ud-lo ('of water, aquatic') sharing a common root with Ancient Greek üllos (ϋλλος) meaning 'fish' or a 'small water snake'. The Illyrian ethnonym shows a dl > ll shift via assimilation as well as the addition of the suffix -uri(o) which is found in Illyrian toponyms such as Tragurium.

Eichner also points out the tribal name's close semantic correspondence to that of the Enchelei which translates to 'eel-people', depicting a similar motif of aquatic snake-like fauna. It is also pointed out that the Ancient Greeks must have learned this name from a tribe in southern Illyria, later applying it to all related and neighbouring peoples.

The terms Illyrians, Illyria and Illyricum have been used throughout history for ethnic and geographic contextualizations that have changed over time. Re-contextualizations of these terms often confused ancient writers and modern scholars. Notable scholarly efforts have been dedicated to trying to analyze and explain these changes.