In Gallo-Roman religion, Loucetios (Latinized as Leucetius) was a Gallic god invariably identified with the Roman Mars. About a dozen inscriptions in his honour have been recovered, mainly from eastern Gaul, with a particular concentration among the Vangiones (a Rhenish tribe). Mars Loucetios is often accompanied by Nemetona. Inscriptions to him have also been found at Bath and Angers.
The name Loucetios may be derived from the Proto-Indo-European root *leuk- ("shine"). It is presumably analogous to OscanLoucetius, "light-bringer," an epithet of Jupiter. The Gaulish and Brythonic forms likely derive from Proto-Celtic*louk(k)et-, "bright, shining, flashing," hence also "lightning," in reference to either a Celtic commonplace metaphor between battles and thunderstorms (Old Irish torannchless, the "thunder feat"), or the divine aura of the hero (the lúan of Cú Chulainn).