The Gauls (Latin: Galli; Ancient Greek: Γαλάται, Galátai) were a group of Celtic peoples of mainland Europe in the Iron Age and the Roman period (roughly 5th century BC to 5th century AD). Their homeland was known as Gaul (Gallia). They spoke Gaulish, a continental Celtic language.

The Gauls emerged around the 5th century BC as bearers of La Tène culture north and west of the Alps. By the 4th century BC, they were spread over much of what is now France, Belgium, Switzerland, Southern Germany, Austria, and the Czech Republic, by virtue of controlling the trade routes along the river systems of the Rhône, Seine, Rhine, and Danube. They reached the peak of their power in the 3rd century BC. During the 4th and 3rd centuries BC, the Gauls expanded into Northern Italy (Cisalpine Gaul), leading to the Roman–Gallic wars, and into the Balkans, leading to war with the Greeks. These latter Gauls eventually settled in Anatolia (contemporary Turkey), becoming known as Galatians.

After the end of the First Punic War, the rising Roman Republic increasingly put pressure on the Gallic sphere of influence. The Battle of Telamon (225 BC) heralded a gradual decline of Gallic power during the 2nd century BC. The Romans eventually conquered Gaul in the Gallic Wars (58–50 BC), making it a Roman province, which brought about the hybrid Gallo-Roman culture.

The Gauls were made up of many tribes (toutās), many of whom built large fortified settlements called oppida (such as Bibracte), and minted their own coins. Gaul was never united under a single ruler or government, but the Gallic tribes were capable of uniting their armies in large-scale military operations, such as those led by Brennus and Vercingetorix. They followed an ancient Celtic religion overseen by druids. The Gauls produced the Coligny calendar.

The ethnonym Galli is generally derived from a Celtic root *gal- 'power, ability' (cf. Old Breton gal 'power, ability', Irish gal 'bravery, courage'). Brittonic reflexes give evidence of an n-stem *gal-n-, with the regular development *galn- > gall- (cf. Middle Welsh gallu, Middle Breton gallout 'to be able', Cornish gallos 'power'). The ethnic names Galátai and Gallitae, as well as Gaulish personal names such as Gallus or Gallius, are also related. The modern French gaillard ('brave, vigorous, healthy') stems from the Gallo-Latin noun *galia- or *gallia- ('power, strength'). Linguist Václav Blažek has argued that Irish gall ('foreigner') and Welsh gâl ('enemy, hostile') may be later adaptations of the ethnic name Galli that were introduced to the British Isles during the 1st millennium AD.

According to Caesar (mid-1st c. BC), the Gauls of the province of Gallia Celtica called themselves Celtae in their own language, and were called Galli in Latin. Romans indeed used the ethnic name Galli as a synonym for Celtae.

Gaulish culture developed over the first millennium BC. The Urnfield culture (c. 1300–750 BC) represents the Celts as a distinct cultural branch of the Indo-European-speaking people. The spread of iron working led to the Hallstatt culture in the 8th century BC; the Proto-Celtic language is often thought to have been spoken around this time. The Hallstatt culture evolved into La Tène culture in around the 5th century BC. The Greek and Etruscan civilizations and colonies began to influence the Gauls, especially in the Mediterranean area.

By the 5th century BC, the tribes later called Gauls had migrated from Central France to the Mediterranean coast. Gallic invaders settled the Po Valley in the 4th century BC, defeated Roman forces in a battle under Brennus in 390 BC, and raided Italy as far south as Sicily.