The Letoon, sometimes Latinized as Letoum, was a sanctuary of Leto near the ancient city Xanthos that was one of the most important religious centres of the Lycian region in Anatolia. The site is located between the towns of Kaş and Fethiye in Antalya province of Turkey, approximately four km south of Xanthos along the Xanthos River. Archaeological finds at the site, which was never a fully occupied settlement, but remained essentially a religious centre, date back to the late sixth century BCE, before the Greek cultural hegemony in Lycia, which began in the early fourth century. In earlier times, the site was probably already sacred to the cult of an earlier mother goddess— she is Eni Mahanahi in Lycia— which was superseded by the worship of Leto, joined by her twin offspring.
In Greek mythology, a claim for an early cult of Apollo in the valley of the Xanthus, unsupported by history or archaeology, was provided by two myths, each connected to an eponymous "Lycus". One sprang from the autochthonous Telchines of Rhodes and would have colonized the region at the time of Deucalion's flood; the other Lycus was an Athenian brother of Aegeus driven from Athens, a seer who introduced the cult of Lycaean Apollo, which a folk etymology connected with Lycia and therefore made him its Athenian colonizer:see Lycus.
The foundations of the Hellenistic temple dedicated to Leto, and her children, Artemis and Apollo, have been excavated under the direction of H. Metzger from 1962. Archæologists have excavated much of the ruins; discoveries include the Letoon trilingual, bearing inscriptions in Greek, Lycian and Aramaic, which has provided crucial keys in the deciphering of the Lycian language; it is conserved in the Fethiye Museum.