Llanddeiniolen is the name of a community area, in the county of Gwynedd, Wales, and is 124 miles (200 km) from Cardiff and 206 miles (332 km) from London. It comprises the hamlet also called Llanddeiniolen, and the villages of Deiniolen, Bethel, Dinorwig, Rhiwlas, Brynrefail and Penisarwaun. It is the third-largest community by population in Gwynedd, with 5072 people in the 2011 census. Also in the community is the dispersed settlement of Fachwen, located on the north shore of Llyn Padarn. The name derives from the Welsh saint Deiniol, first bishop of Bangor or from his son, saint Deiniolen.

Deiniolen is a village in Gwynedd, Wales, at the foot of Elidir Fawr, in Llanddeiniolen Community. Deiniolen has views over Caernarfon and on a clear day, Holyhead Mountain and occasionally the Wicklow Mountains in Ireland, can be seen. The population of the electoral ward was 1,909 as of 2011, including nearby Dinorwig, with 81.2% of the population speaking Welsh. The community, Llanddeiniolen, has a population of 5,072 as of the 2011 census, the third-largest in Gwynedd.