The aos sí (Irish pronunciation: [iːs ˈʃiː], older form aes sídhe [eːs ˈʃiːə]) are a supernatural race in Irish mythology and Scottish mythology comparable to the fairies or elves. They are said to live underground in the fairy mounds, across the western sea, or in an invisible world that coexists with the world of humans. This world is described in the Book of Invasions (recorded in the Book of Leinster) as a parallel universe in which the aos sí walk amongst the living. In the Irish language, aos sí means "people of the mounds" (the mounds are known in Irish as "the sídhe"). In Irish literature the people of the mounds are also called daoine sídhe [ˈdiːnʲə ˈʃiːə]; in Scottish mythology they are daoine sìth. They are variously said to be the ancestors, the spirits of nature, or goddesses and gods.

Some secondary and tertiary sources erroneously refer to aos sí simply as "the sídhe" (lit.: mounds).^{[citation needed]}

In many Gaelic tales the aos sí are later, literary versions of the Tuatha Dé Danann ("People of the Goddess Danu") – the deities and deified ancestors of Irish mythology. Some sources describe them as the survivors of the Tuatha Dé Danann who retreated into the Otherworld after they were defeated by the Milesians – the mortal Sons of Míl Espáine who, like many other early invaders of Ireland, came from Spain. Geoffrey Keating, an Irish historian of the late 17th century, equates Spain with the Land of the Dead.