Llanfair-Nant-Gwyn is a hamlet and small parish in the community of Eglwyswrw, north Pembrokeshire, Wales.

An English translation of Llanfair-Nant-Gwyn would be "St Mary's church of the white stream".

According to Lewis, the parish derives its name "from the dedication of its church to St.Mary, and its distinguishing adjunct probably from the abundance of white quartz stones scattered over the lands and in the bed of a brook by which it is watered".

[1][3] R. J. Withers designed the parish church when it was rebuilt in 1855; it has a wooden, slated spire[4] and is dedicated to St Mary.

The name of the stream running from Meigan Wells, past Pant-y-Deri Farm, to join the River Nevern shortly before Nant Gwyn is not named on modern maps, but George Owen (1594)[6] describes the two streams thus: "the Nevern... ...receaveth from the north-east the brooke called yr Amelh whose spring is above St. Meigans, whence it runneth right west and neere Nantgwyn Chappel, receaveth the cleere rillet, called Nant-Gwyn, then hastening toward Jordan's Mill from the south-east..."St Meigan is linked to a fair in Eglwyswrw to the west, and Pant-y-Deri hosted a rock concert called Meigan's Fayre in the 1970s.