Gaulish is an extinct Celtic language spoken in parts of Continental Europe before and during the period of the Roman Empire. In the narrow sense, Gaulish was the language of the Celts of Gaul (now France, Luxembourg, Belgium, most of Switzerland, Northern Italy, as well as the parts of the Netherlands and Germany on the west bank of the Rhine). In a wider sense, it also comprises varieties of Celtic that were spoken across much of central Europe ("Noric"), parts of the Balkans, and Anatolia ("Galatian"), which are thought to have been closely related. The more divergent Lepontic of Northern Italy has also sometimes been subsumed under Gaulish.

Together with Lepontic and the Celtiberian spoken in the Iberian Peninsula, Gaulish is a member of the geographic group of Continental Celtic languages. The precise linguistic relationships among them, as well as between them and the modern Insular Celtic languages, are uncertain and a matter of ongoing debate because of their sparse attestation.

Gaulish is found in some 800 (often fragmentary) inscriptions including calendars, pottery accounts, funeral monuments, short dedications to gods, coin inscriptions, statements of ownership, and other texts, possibly curse tablets. Gaulish was first written in Greek script in southern France and in a variety of Old Italic script in northern Italy. After the Roman conquest of those regions, writing shifted to Latin script. During his conquest of Gaul, Caesar reported that the Helvetii were in possession of documents in the Greek script, and all Gaulish coins used the Greek script until about 50 BC.

Gaulish in Western Europe was supplanted by Vulgar Latin. It is thought to have been a living language well into the 6th century.

The legacy of Gaulish may be observed in the modern French language and the Gallo-Romance languages, in which 150–400 words, mainly referring to pastoral and daily activities, are known to be derived from the extinct Continental Celtic language. Following the 1066 Norman Conquest, some of these words have also entered the English language, through the influence of Old French.

It is estimated that during the Bronze Age, Proto-Celtic started splitting into distinct languages, including Celtiberian and Gaulish. Due to the expansion of Celtic tribes in the 4th and 3rd centuries BC, closely related forms of Celtic came to be spoken in a vast arc extending from Britain and France through the Alpine region and Pannonia in central Europe, and into parts of the Balkans and Anatolia. Their precise linguistic relationships are uncertain due to fragmentary evidence.

The Gaulish varieties of central and eastern Europe and of Anatolia (called Noric and Galatian, respectively) are barely attested, but from what little is known of them it appears that they were quite similar to those of Gaul and can be considered dialects of a single language. Among those regions where substantial inscriptional evidence exists, three varieties are usually distinguished.

The relationship between Gaulish and the other Celtic languages is also debated. Most scholars today agree that Celtiberian was the first to branch off from other Celtic. Gaulish, situated in the centre of the Celtic language area, shares with the neighboring Brittonic languages of Britain, as well as the neighboring Italic Osco-Umbrian languages, the change of the Indo-European labialized voiceless velar stop /kʷ/ > /p/, while both Celtiberian in the south and Goidelic in Ireland retain /kʷ/. Taking this as the primary genealogical isogloss, some scholars divide the Celtic languages into a "q-Celtic" group and a "p-Celtic" group, in which the p-Celtic languages Gaulish and Brittonic form a common "Gallo-Brittonic" branch. Other scholars place more emphasis on shared innovations between Brittonic and Goidelic and group these together as an Insular Celtic branch. Sims-Williams (2007) discusses a composite model, in which the Continental and Insular varieties are seen as part of a dialect continuum, with genealogical splits and areal innovations intersecting.