Ancient Celtic religion, commonly known as Celtic paganism, was the religion of the ancient Celtic peoples of Europe. Because the ancient Celts did not have writing, evidence about their religion is gleaned from archaeology, Greco-Roman accounts (some of it hostile and probably not well-informed), and literature from the early Christian period. Celtic paganism was one of a larger group of Iron Age polytheistic religions of Europe. It varied by region and over time, but underlying this were "broad structural similarities" and "a basic religious homogeneity" among the Celtic peoples.

The names of over two hundred Celtic deities have survived (see list of Celtic deities), although it is likely that many of these were alternative names, regional names or titles for the same deity. Some deities were venerated only in one region, but others were more widely known. Deities found in many regions include Lugus, the tribal god Toutatis, the thunder god Taranis, the horned god Cernunnos, the horse and fertility goddess Epona, the divine son Maponos, as well as Belenos, Ogmios, and Sucellos. Celtic healing deities were often associated with sacred springs. Caesar says the Gauls believed they all descended from a god of the dead and underworld. Triplicity is a common theme, with a number of deities seen as threefold, for example the Three Mothers. Some figures from medieval Irish mythology have been interpreted as iterations of earlier deities. According to Miranda Aldhouse-Green, the Celts were also animists, believing that every part of the natural world had a spirit.

The priests of Celtic religion were "magico-religious specialists" called druids, but little is definitively known about them. Greco-Roman writers said the Celts held ceremonies in sacred groves and other natural shrines, called nemetons, while some Celtic peoples also built temples or ritual enclosures. Celtic peoples often made votive offerings: treasured items deposited in water and wetlands, or in ritual shafts and wells. There is evidence that ancient Celtic peoples sacrificed animals, almost always livestock or working animals. There is also some evidence that ancient Celts sacrificed humans, and some Greco-Roman sources claim the Gauls sacrificed criminals by burning them in a wicker man.

It is not clear what religious festivals the ancient Celts held, but the Insular Celtic peoples celebrated four seasonal festivals, known to the medieval Gaels as Beltaine (1 May), Lughnasadh (1 August), Samhain (1 November) and Imbolc (1 February).

After the Roman Empire's conquest of Gaul (58–51 BC) and southern Britain (43 AD), Celtic religion there underwent some Romanisation, resulting in a syncretic Gallo-Roman religion with deities such as Lenus Mars, Apollo Grannus, and Telesphorus.

The Gauls gradually converted to Christianity from the third century onward. After the end of Roman rule in Britain (c.410 AD) Celtic paganism began to be replaced by Anglo-Saxon paganism over much of what became England. The Celtic populations of Britain and Ireland gradually converted to Christianity from the fifth century onward. However, Celtic paganism left a legacy in many of the Celtic nations, influenced mythology and in the 20th century served as the basis for a new religious movement, Celtic Neopaganism.