Esus is a Celtic god known from iconographic, epigraphic, and literary sources.

The 1st-century CE Roman poet Lucan's epic Pharsalia mentions Esus, Taranis, and Teutates as gods to whom the Gauls sacrificed humans. This rare mention of Celtic gods under their native names in a Greco-Roman text has been the subject of much comment. Almost as often commented on are the scholia to Lucan's poem (early medieval, but relying on earlier sources) which tell us the nature of these sacrifices: in particular, that Esus's victims were suspended from a tree and bloodily dismembered. The nature of this ritual is obscure, but it has been compared with a wide range of sources, including Welsh and Germanic mythology, as well as with the violent end of the Lindow Man.

Esus has been connected (through an inscription which identifies him and an allied character, Tarvos Trigaranos, by name) with a pictorial myth on the Pillar of the Boatmen, a Gallo-Roman column from Paris. This myth associates Esus, felling or pruning a tree, with a bull and three cranes. A similar monument to Esus and Tarvos Trigaranos from Trier confirms this association. The nature of this myth is little understood; it at least confirms the scholia's association of Esus with trees.

Esus appears rarely in inscriptions, with only two certain attestations of his name in the epigraphic record. His name appears more commonly as an element of personal names. While Lucan only attributes the worship of Esus to unspecified Gauls, inscriptions place the worship of Esus in Gaul, Noricum, and perhaps Roman North Africa; personal names may also place his worship in Britain. In inscriptions, Esus is attested as early as the 1st century BCE. In Latin literature, he may appear as late as the 5th century CE.

A large number of etymologies have been proposed for the name "Esus". The nature of the god's name is not certain. Wolfgang Meid [de] has suggested it may be a euphemism, cover-name, or epithet of the god. Claude Sterckx [fr] has even questioned whether "Esus" was a name given to only one deity (though his view is a minority one).

The most widely adopted etymology derives Esus's name from the proto-Indo-European verbal root *h₁eis- ("to be reverent, to worship"), cognate with Italic aisos ("god"). This etymology is supported by the fact that it makes the initial vowel of Esus's name long, which agrees with both Lucan's poetic stress and the variant spellings which use "ae" for this vowel. However, D. Ellis Evans points out that the more common etymology for Italic aisos derives this word from an Etruscan word; since Etruscan is non-Indo-European and Celtic is Indo-European, this would rule out a relationship between Esus and aisos.

Joseph Vendryes linked the name with proto-Indo-European *esu- ("good"). Jan de Vries is sceptical of this, pointing out that this is difficult to reconcile with the fearful god described in Lucan and the scholia. Meid suggests the name would then be a euphemism, comparing it with the Irish god-name Dagda ("the good god"). Henri d'Arbois de Jubainville linked it to proto-Indo-European *is- ("to wish"). T. F. O'Rahilly linked it to proto-Indo-European *eis- ("vital force, life"). Félix Guirand suggested the name was cognate with Latin erus ("lord", "master"), which Meid notes is a common epiclesis given to deities (Freyr, Ba'al). Other etymologies have variously connected the name with German Ehre ("honour"), Ancient Greek αἰδέομαι (aidéomai, "to be ashamed"), Old Norse eir ("brass, copper"), and Breton heuzuz ("terrible")

Lucan's Pharsalia or De Bello Civili (On the Civil War) is an epic poem, begun about 61 CE, on the events of Caesar's civil war (49–48 BCE). The passage relevant to Esus occurs in "Gallic excursus", an epic catalogue detailing the rejoicing of the various Gaulish peoples after Julius Caesar removed his legions from Gaul (where they were intended to control the natives) to Italy. The passage thus brings out two themes of Lucan's work, the barbarity of the Gauls and the unpatriotism of Caesar.