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.

The Welsh placename (shortened from Castell Fuwch) means "Cow castle", and is perhaps a mocking name for an abandoned fortification, inhabited only by cows.

There are a few English placenames in the southern part of the parish,[2] but there is no evidence to suggest large-scale English colonisation in the medieval period, and the parish has always been essentially Welsh-speaking.

The church of St Michael was rebuilt in 1875 to the designs of Edwin Dolby.