The Llŷn Peninsula (Penrhyn Llŷn or Pen Llŷn, ɬɨːn) extends into the Irish Sea from North West Wales, south west of the Isle of Anglesey. It is part of the historic county of Caernarfonshire, and historic region and local authority area of Gwynedd. Much of the eastern part of the peninsula, around Criccieth, may be regarded as part of Eifionydd rather than Llŷn, although the boundary is somewhat vague. The area of Llŷn is about , and its population is at least 20,000.
Historically, the peninsula was travelled by pilgrims en route to Bardsey Island (Welsh: Ynys Enlli), and its relative isolation has helped to conserve the Welsh language and culture, for which the locality is now famous. This perceived remoteness from urban life has lent the area an unspoilt image which has made Llŷn a popular destination for both tourists and holiday home owners. Holiday homes remain contentious among locals, many of whom feel they are priced out of the housing market by incomers. From the 1970s to the 1990s, a group known as Meibion Glyndŵr claimed responsibility for several hundred arson attacks on holiday homes using incendiary devices, some of which took place in Llŷn.
The Llyn Area of Outstanding Natural Beauty covers approximately .
The name Llŷn is also sometimes spelled Lleyn, although this spelling is now less common. The name is thought to be of Irish origin, and to have the same root Laigin (Laighin) in Irish as the word Leinster and which also occurs in Porth Dinllaen on the north coast.
Following the death of Owain Whitetooth (Ddantgwyn), king of Gwynedd, Owain's son Saint Einion seems to have ruled Llŷn as a kingdom separate from his brother Cuneglas' kingdom in Rhos. He is credited with having sponsored Saint Cadfan's monastery on Bardsey Island, which became a major centre of during medieval times. There are numerous wells throughout the peninsula, many dating back to the pre-Christian era. Many have holy connotations and they were important stops for pilgrims heading to the island.