The Demetae were a Celtic people of Iron Age Britain who inhabited modern Pembrokeshire and Carmarthenshire in south-west Wales, and gave their name to the county of Dyfed. Their origin is uncertain, however, a number of place names are similar to what were Celtic regions in what is now the Bordeaux region of France as Llanmadoc and Landes du Médoc, Gwynedd and Gironde, Demetae and Devèze, suggestive of Pre-Roman Celtic Northerly travels on the portside of the North Atlantic Current.
They are mentioned in Ptolemy's Geographia, as being west of the Silures. He mentions two of their towns, Moridunum (modern Carmarthen) and Luentinum (identified as the Dolaucothi Gold Mines near Pumsaint, Carmarthenshire). They are not mentioned in Tacitus' accounts of Roman warfare in Wales, which concentrate on their neighbours the Silures and Ordovices.
Vortiporius, "tyrant of the Demetae", is one of the kings condemned by Gildas in his 6th century polemic De Excidio et Conquestu Britanniae. This probably signifies the sub-Roman petty kingdom of Dyfed.