Proto-Celtic religion refers to the belief systems attributed to the speakers of the Proto-Celtic language, and encompasses mythological themes, legendary narratives, folk traditions and cosmological concepts that can be reconstructed for early Celtic culture. Proto-Celtic is generally dated to the Late Bronze Age (), and any reconstruction of Proto-Celtic religion therefore predates the historically attested religions of the Ancient Celts.
Through the comparative method, Celtic philologists and historical linguists have proposed reconstructions of deities, mythic figures, ritual concepts, and place-names, with varying degrees of scholarly confidence (reconstructed forms are conventionally marked with an asterisk). These reconstructions draw primarily on linguistic evidence and comparative analysis, and are supplemented by later literary, epigraphic, and archaeological sources. Modern scholarship therefore stresses methodological restraint, treating Proto-Celtic religion as a constellation of related traditions rather than a fully reconstructible, homogeneous belief system.
According to linguist John T. Koch, the period is appropriate for the existence of a unified, and possibly geographically expansive, Proto-Celtic language. The Proto-Celtic homeland is generally associated in scholarship with the Urnfield culture (<abbr></abbr>) and the early Hallstatt Iron Age (<abbr></abbr>) in Central Europe.
The period around 900 BC is commonly identified with the breakup of Proto-Celtic into distinct branches (Hispano-Celtic versus Gallo-Brittonic and Goidelic, or Continental versus Insular Celtic).
The beliefs of the Proto-Celtic speakers cannot be reconstructed as a coherent or unified system, as no direct descriptions or indigenous religious writings survive from the Proto-Celtic period. Reconstruction instead relies on a combination of indirect sources from later periods, including Classical accounts by Greek and Roman authors (such as Caesar, Strabo, and Lucan), ancient inscriptions (notably theonyms and dedicatory formulas), early medieval Irish and Welsh literature, archaeological evidence, and comparative Indo-European linguistics. These sources differ widely in date, genre, and cultural context, and many are shaped by external perspectives or later Christian reinterpretation, providing fragmentary insights into inherited beliefs and practices rather than a systematic theology. Several motifs traditionally described as "Celtic" are also attested in other European traditions, particularly Germanic, suggesting that some elements may reflect shared Indo-European traditions rather than uniquely Celtic religious concepts.
Aspects of Proto-Celtic religion are reconstructed by identifying recurrent patterns across regions and periods. Where semantic continuity can be reasonably established, this also involves comparing cognate religious terms preserved in different Celtic languages. Some elements are considered among the more securely reconstructed aspects of Proto-Celtic paganism, particularly the sacral role of poets (*bardos), druids (*druwides), and seers (*wÃÂtis), as well as the recurrence of certain shared divine names (notably *Lugus). A number of shared concepts likewise appear to have been closely tied to the natural environment, including sacred groves (*nemetom), mountains, lakes, and springs. Despite these common elements, claims of pan-Celtic uniformity are therefore treated with caution in Celtic scholarship, which instead points to a "tangible relationship based upon common inheritance" rather than a fully unified religious system.
The reconstructions presented in this section are proposed by historical linguists and philologists on the basis of the comparative method, which infers earlier linguistic forms through systematic comparison of cognate evidence across related Celtic languages and, where relevant, the wider Indo-European linguistic family. Reconstructed forms are conventionally marked with an asterisk (*), while forms whose attribution to the Proto-Celtic period is uncertain are preceded by a question mark (?).
In the case of religious terminology and mythological motifs, reconstruction is often complicated by the long chronological gap between the Proto-Celtic period (), the earliest available evidence of ancient Celtic religious beliefs (mainly from the late 1st millennium BC to the early 1st millennium AD), and the medieval Celtic textual traditions ().