Llanystumdwy is a predominantly Welsh-speaking village, community and electoral ward on the Llŷn Peninsula in Wales. It lies in the traditional county of Caernarfonshire but is currently administered as part of the unitary authority of Gwynedd. It is not regarded as being part of Llŷn, but as belonging instead to the ancient commote of Eifionydd on the Cardigan Bay coast, where it has its own beach. The community includes the villages of Chwilog, Afon Wen, Llanarmon, and Llangybi, plus the hamlets of Rhoslan and Pencaenewydd.

The Llŷn Peninsula extends 30 miles (50 km) 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 400 km2 (150 sq mi), and its population is at least 20,000.