Hades (/ˈheɪdiːz/; Ancient Greek: ᾍδης, romanized: Hā́idēs, Attic Greek: [háːi̯dεːs], later [háːdeːs]), in the ancient Greek religion and mythology, is the God of the dead and riches and the King of the underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea, although this also made him the last son to be regurgitated by his father. He and his brothers, Zeus and Poseidon, defeated, overthrew, and replaced their father's generation of gods, the Titans, and claimed joint sovereignty over the cosmos. Hades received the underworld, Zeus the sky, and Poseidon the sea, with the solid earth, which was long the domain of Gaia, available to all three concurrently. In artistic depictions, Hades is typically portrayed holding a bident and wearing his helm with Cerberus, the three-headed guard-dog of the underworld, standing at his side.

Roman-era mythographers eventually equated the Etruscan god Aita, and the Roman gods Dis Pater and Orcus, with Hades, and merged all these figures into Pluto, a Latinisation of Plouton (Ancient Greek: Πλούτων, romanized: Ploútōn), itself a euphemistic title (meaning "the rich one") often given to Hades.

The origin of Hades's name is uncertain but has generally been seen as meaning "the unseen one" since antiquity. An extensive section of Plato's dialogue Cratylus is devoted to the etymology of the god's name, in which Socrates is arguing for a folk etymology not from "unseen" but from "his knowledge (eidenai) of all noble things". Modern linguists have proposed the Proto-Greek form *Awides ("unseen"). The earliest attested form is Aḯdēs (Ἀΐδης), which lacks the proposed digamma. Martin Litchfield West argues instead for an original meaning of "the one who presides over meeting up" from the universality of death.

In Homeric and Ionic Greek, he was known as Áïdēs. Other poetic variations of the name include Aïdōneús (Ἀϊδωνεύς) and the inflected forms Áïdos (Ἄϊδος, gen.), Áïdi (Ἄϊδι, dat.), and Áïda (Ἄϊδα, acc.), whose reconstructed nominative case *Áïs (*Ἄϊς) is, however, not attested. The name as it came to be known in classical times was Háidēs (Ἅιδης). Later the iota became silent, then a subscript marking (ᾍδης), and finally omitted entirely (Άδης).

Perhaps from fear of pronouncing his name, around the 5th century BC, the Greeks started referring to Hades as Plouton (Πλούτων, Ploútōn, [ˈpluː.tɔːn]), with a root meaning "wealthy", considering that from the abode below (i.e., the soil) come riches (e.g., fertile crops, metals and so on). Plouton became the Roman god who both rules the underworld and distributed riches from below. This deity was a mixture of the Greek god Hades and the Eleusinian icon Ploutos, and from this he also received a priestess, which was not previously practiced in Greece. More elaborate names of the same genre were Ploutodótēs (Πλουτοδότης, [pluː.toˈdo.tεːs]) or Ploutodotḗr (Πλουτοδοτήρ, [pluː.to.doˈtεːr]), meaning "giver of wealth".

Epithets of Hades include Agesander (Ἀγήσανδρος, [aˈgεː.san.dros]) and Agesilaos (Ἀγεσίλαος, [a.geˈsi.la.os]), both from ágō (ἄγω, "lead", "carry" or "fetch") and anḗr (ἀνήρ, "man") or laos (λαός, "men" or "people"), describing Hades as the god who carries away all. Nicander uses the form Hegesilaus (Ἡγεσίλαος, [hεː.geˈsi.la.os]).

He was also referred to as Zeus katachthonios (Ζεὺς καταχθόνιος, [zdeu̯s ka.taˈkʰtʰo.ni.os]), meaning "the Zeus of the underworld", by those avoiding his actual name, as he had complete control over the underworld.

In Greek mythology, Hades, the god of the Greek underworld, was the first-born son of the Titans Cronus and Rhea. He had three older sisters, Hestia, Demeter, and Hera, as well as a younger brother, Poseidon, all of whom had been swallowed whole by their father as soon as they were born. Zeus was the youngest child and through the machinations of their mother, Rhea, he was the only one that had escaped this fate. Upon reaching adulthood, Zeus managed to force his father to disgorge his siblings. After their release, the six younger gods, along with allies they managed to gather, challenged the elder gods for power in the Titanomachy, a divine war. Armed with the helm of invisibility forged for him by the Cyclopes, Hades with his siblings and other divine allies defeated the Titans and became rulers in their place The war lasted for ten years and ended with the victory of the younger gods. Following their victory, according to a single famous passage in the Iliad (Book XV, ln.187–93), Hades and his two brothers, Poseidon and Zeus, drew lots for realms to rule. Zeus received the sky, Poseidon received the seas, and Hades received the underworld, the unseen realm to which the souls of the dead go upon leaving the world as well as any and all things beneath the earth.