Segesta (Greek: Ἔγεστα, Egesta, or Σέγεστα, Ségesta, or Αἴγεστα, Aígesta; Sicilian: Siggésta) was one of the major cities of the Elymians, one of the three indigenous peoples of Sicily. The other major cities of the Elymians were Eryx and Entella. It is located in the northwestern part of Sicily in Italy, near the modern commune of Calatafimi-Segesta in the province of Trapani. The hellenization of Segesta happened very early and had a profound effect on its people.

The origin and foundation of Segesta is extremely obscure. The tradition current among the Greeks and adopted by Thucydides, ascribed its foundation to a band of Trojan settlers, fugitives from the destruction of their city; and this tradition was readily welcomed by the Romans, who in consequence claimed a kindred origin with the Segestans. Thucydides seems to have considered the Elymians (Latin: Elymi), a barbarian tribe in the neighborhood of Eryx and Segesta, as descended from the Trojans in question; but another account represents the Elymi as a distinct people, already existing in this part of Sicily when the Trojans arrived there and founded the two cities. A different story seems also to have been current, according to which Segesta owed its origin to a band of Phocians, who had been among the followers of Philoctetes; and, as usual, later writers sought to reconcile the two accounts.

Another version of the Trojan story related in Virgil's Aeneid, which would seem to have been adopted by the inhabitants themselves, ascribed the foundation of the city jointly by the territorial king Egestus or Aegestus (the Acestes of Virgil), who was said to be the offspring of a Dardanian damsel named Segesta by the river god Crinisus, and by those of Aeneas' folk who wished to remain behind with Acestes to found the city of Acesta. We are told also that the names of Simois and Scamander were given by the Trojan colonists to two small streams which flowed beneath the town, and the latter name is mentioned by Diodorus Siculus as one still in use at a much later period.

The belief that the name of the city was originally Acesta or Egesta and changed to Segesta by the Romans to avoid its ill-omened meaning in Latin (Ægesta in Latin means "excements") is disproved by coins which prove that considerably before the time of Thucydides it was called by the inhabitants themselves Segesta, though this form seems to have been softened by the Greeks into Egesta.

The city was occupied by a people distinct from the Sicanians, the native race of this part of Sicily, and on the other that it was not a Greek colony. Thucydides, in enumerating the allies of the Athenians at the time of the Peloponnesian War, distinctly calls the Segestans barbarians. At the same time they appear to have been, from a very early period, in close connection with the Greek cities of Sicily, and entering into relations both of hostility and alliance with the Hellenic states, wholly different from the other barbarians in the island. The early influence of Greek civilisation is shown also by their coins, which are inscribed with Greek characters, and bear the unquestionable impress of Greek art.

On a hill just outside the site of the ancient city of Segesta lies an unusually well preserved Doric temple. It is thought to have been built in the 420s BC by an Athenian architect, despite the city not having a large Greek population. The temple has six by fourteen columns on a base measuring 21 by 56 meters, on a platform three steps high. Several elements suggest that the temple was never finished. The columns have not been fluted as they normally would have been in a Doric temple and there are still bosses present in the blocks of the base (used for lifting the blocks into place but then normally removed). The temple also lacks a cella, any ornamentation, altar or deity dedication, and was never roofed over. The temple escaped destruction by the Carthaginians in the late 5th century.