The Kingdom of Dyfed was a sub-Roman and early medieval kingdom in south-west Wales.
Dyfed, or in its Latin form Demetia, was one of the ancient kingdoms of Wales prior to the Norman Conquest. It succeeded to the former Roman administrative civitas of the Demetae tribe. It originally occupied an area roughly corresponding to modern Pembrokeshire.
In the 10th century Hywel Dda united Dyfed and the neighbouring kingdom of Seisyllwg under his rule. The new kingdom became known as Deheubarth and covered an area roughly corresponding to modern Dyfed.