Letoon or Letoum (Turkish: Letoon, Ancient Greek: Λητῷον) in the Fethiye district of Muğla Province, Turkey, was a sanctuary of Leto located 4 kilometres (2.5 mi) south of the ancient city of Xanthos, to which it was closely associated, and along the Xanthos River. It was one of the most important religious centres in the region though never a fully-occupied settlement.
Letoon was added as a UNESCO World Heritage Site in 1988.
Xanthos or Xanthus, also referred to by scholars as Arna, its Lycian name, (Turkish: Ksantos, Lycian: 𐊀𐊕𐊑𐊏𐊀 Arñna, Greek: Ξάνθος, Latin: Xanthus) was an ancient city near the present-day village of Kınık, in Antalya Province, Turkey. The ruins are located on a hill on the left bank of the River Xanthos. The number and quality of the surviving tombs at Xanthos are a notable feature of the site, which, together with nearby Letoon, was declared to be a UNESCO World Heritage Site in 1988.
The city of Xanthos was a centre of culture and commerce for the Lycians, and later for the Persians, Greeks and Romans who in turn conquered the region. Xanthos influenced its neighbours architecturally; the Nereid Monument directly inspired the Mausoleum at Halicarnassus in the region of Caria.