Celtic Polytheism, sometimes known as Celtic Paganism, refers to the religious beliefs and practises of the ancient Celtic peoples of western Europe prior to Christianisation.

Celtic polytheism, as its name suggests, was polytheistic, believing in a number of different deities, and was also animistic, believing in spirits existing in natural objects such as trees and rocks. Religious beliefs and practises of the Celts varied throughout the different Celtic lands, which included Ireland, Britain, Celtiberia, Gaul, areas along the Danube river, and Galatia; however there were commonalities shared by all.

Celtic religious practices bear the marks of Romanization following the Roman Empire's conquest of certain Celtic lands such as Gaul (58–51 BCE) and Britain (43 CE), although the depth and significance of Romanization is a subject of scholarly disagreement.

Celtic polytheism declined in the Roman Empire period, especially after the outlawing of one form of it, Druidism, by the emperor Claudius in 54 CE. It persisted somewhat longer in Britain and Ireland, where it gradually disappeared during Christianization, over the 5th to 6th centuries.

Celtic religion was polytheistic, believing in many deities, both gods and goddesses. The most notable of these were pan-Celtic, being worshipped across much of the Celtic world, albeit under various regional names and with different associations. Despite the notability of these pan-Celtic deities, they make up only a tiny percentage of Celtic gods; out of the roughly 300 Celtic deities that we know about, only around 60 can be found in more than one region, and of those, only about 20–30 are pan-Celtic.

The Celts were also animists, believing in deities existing in most aspects of nature, such as in trees and streams, who were often venerated at local shrines.

According to classical era sources, the Celts worshipped the forces of nature and did not envisage deities in anthropomorphic terms as other pagan peoples such as the Greeks, Romans, and Egyptians did. This appeared to change as the classical peoples grew in influence over the Celtic cultures, as the Celts did begin to give their deities human forms, and they moved from a more animistic-based faith to a more Romanized polytheistic view.

Several of these deities, including Lugus and Matrones, exhibited triplism, being found in a set of three.

 Various Neopagan groups claim association with Celtic polytheism. These groups range from the Reconstructionists, who work to practice ancient Celtic religion with as much accuracy as possible; to new age, eclectic groups who take some of their inspiration from Celtic mythology and iconography but place little significance on any sort of historical precedent, the most notable of which is Neo-druidry.