Llanystumdwy [ɬanɪstɪmdʊɨ] 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.