Llanystumdwy (Welsh pronunciation: [ɬanəstɪmdʊɨ̯]) is a predominantly Welsh-speaking village, community and electoral ward in the Eifionydd area of Gwynedd in Wales. The village lies on the southern coast of the Llŷn Peninsula, with a beach facing Cardigan Bay, between Criccieth and Pwllheli at the point where the A497 road crosses the Afon Dwyfor.
The community and ward includes the villages of Chwilog, Afon Wen, Llanarmon, and Llangybi, plus the hamlets of Rhoslan and Pencaenewydd. It is represented by a county councillor on Gwynedd Council, and had a population of 1,919 in 2021. Historically it was part of Caernarfonshire, until the creation of Gwynedd in 1974.