The Llŷn Peninsula (Welsh: Penrhyn Llŷn or Pen Llŷn, Welsh pronunciation: [ɬɨːn]) is a peninsula in Gwynedd, Wales, with an area of about 400 km (150 sq mi), and a population of at least 20,000. It extends 30 miles (50 km) into the Irish Sea, and its southern coast is the northern boundary of the Tremadog Bay inlet of Cardigan Bay. The peninsula was a cantref within the medieval kingdom of Gwynedd, and became part of Caernarfonshire from 1284 until that county was abolished for administrative purposes in 1974. It borders Arfon and Eifionydd to the east, but the boundary is vague.
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.