Castlebythe (Welsh: Cas-fuwch) is a village and parish in Pembrokeshire, Wales, on the southern slopes of the Preseli Hills, 10 km south-east of Fishguard. The northern part of the parish is in the Pembrokeshire Coast National Park. Together with the parishes of Henry's Moat, Little Newcastle, Morvil and Puncheston, it constitutes the community of Puncheston.