Anglesey (island) or Ynys Môn (island), island, located in the Irish Sea off the coast of north-west Wales, and separated from the mainland county of Gwynedd by the narrow Menai Strait. Anglesey, also known as Anglesea, is the largest island in England and Wales; Holy Island lies off Anglesey’s north-west coast and is connected to the island by road and rail bridges. The area of Anglesey island is 676 sq km (261 sq mi) and almost the entire 201-km (125-mi) coastline has been designated an Area of Outstanding Natural Beauty by the Countryside Agency. In addition, two stretches of coastline are designated as heritage coast—the North Anglesey Heritage Coast in the north and Aberffraw Bay Heritage Coast in the south-west. Anglesey and Holy Island together constitute the Isle of Anglesey unitary authority.

Anglesey’s main towns are Amlwch (1991, 3,519) on the north coast, Menai Bridge (1991, 3,175) and Beaumaris (1991, 1,561) on the south coast, and Llangefni (1991, 4,643), which is located in central Anglesey and is the administrative centre of the unitary authority. The island is linked to mainland Wales by the Menai Bridge, a suspension bridge built by Scottish civil engineer Thomas Telford between 1819 and 1826.