Llanuwchllyn is a village and community in Gwynedd, Wales, near the southern end of Bala Lake (Llyn Tegid). Its population according to the United Kingdom Census 2001 was 834, of whom about 81% were Welsh-speaking.
The parish church of St Deiniol is a grade II* listed building. 
The village was the birthplace of Welsh language author and educationalist Owen Morgan Edwards.
Caer gai, a Roman fort near Llanuwchllyn, was traditionally known as the home of Cei, the character in the Arthurian legend known in English as Sir Kay. Poets of the 15th century recorded a story, ultimately deriving from the Prose Merlin included in the Lancelot-Grail and the Post-Vulgate Cycle, that King Arthur and Cei were brought up at Caer-gai as foster brothers. Caer gai is also grade II* listed. 
- Llanuwchllyn Census 2001 National Office of Statistics
- Welsh-speaking statistics Welsh Language Board
- "Parish Church of St Deiniol, Llanuwchllyn". British Listed Buildings. Retrieved 17 December 2013.
- Bromwich, p. 311.
- "Caer Gai, including adjoining forecourt walls to the NE, Llanuwchllyn". British Listed Buildings. Retrieved 17 December 2013.
- Bromwich, Rachel (2006). Trioedd Ynys Prydein: The Triads of the Island of Britain. University Of Wales Press. ISBN 0-7083-1386-8.