The Celtic languages (usually , but sometimes ) are a group of related languages descended from Proto-Celtic. They form a branch of the Indo-European language family. The term "Celtic" was first used to describe this language group by Edward Lhuyd in 1707, following Paul-Yves Pezron, who made the explicit link between the Celts described by classical writers and the Welsh and Breton languages.

During the 1st millennium BC, Celtic languages were spoken across much of Europe and central Anatolia. Today, they are restricted to the northwestern fringe of Europe and a few diaspora communities. There are six living languages: the four continuously living languages Breton, Irish, Scottish Gaelic and Welsh, and the two revived languages Cornish and Manx. All are minority languages in their respective countries, though there are continuing efforts at revitalisation. Welsh is an official language in Wales and Irish is an official language of Ireland and of the European Union. Welsh is the only Celtic language not classified as endangered by UNESCO. The Cornish and Manx languages went extinct in modern times. They have been the object of revivals and now each has several hundred second-language speakers.

Irish, Manx and Scottish Gaelic form the Goidelic languages, while Welsh, Cornish and Breton are Brittonic. All of these are Insular Celtic languages, since Breton, the only living Celtic language spoken in continental Europe, is descended from the language of settlers from Britain. There are a number of extinct but attested continental Celtic languages, such as Celtiberian, Galatian and Gaulish. Beyond that there is no agreement on the subdivisions of the Celtic language family. They may be divided into P-Celtic and Q-Celtic.

The Celtic languages have a rich literary tradition. The earliest specimens of written Celtic are Lepontic inscriptions from the 6th century BC in the Alps. Early Continental inscriptions used Italic and Paleohispanic scripts. Between the 4th and 8th centuries, Irish and Pictish were occasionally written in an original script, Ogham, but Latin script came to be used for all Celtic languages. Welsh has had a continuous literary tradition from the 6th century AD.