Penarth Fawr is a hall house in the community of Llanystumdwy, Gwynedd, Wales. The oldest part of the dates to the mid-fifteenth century and consists of a three-bay hall and a one-bay service end to the south; a parlour wing to the north has been demolished. There is a seventeenth-century wing attached to the rear of the building. The original house has been altered several times, but was restored to approximately its medieval appearance in 1937 and is notable for its intact medieval roof and screen.