A cantref, or geographical unit in medieval Wales.
Supposedly named after an early sixth century king, Gwynllyw Farfog ap Glywys, Gwynllwg essentially comprised the coastal plain stretching between the rivers Rhymney and Usk and was one of the cantrefi that comprised the Welsh medieval kingdom of Glywysing, later known as Morgannwg.
After the defeat and death of Caradog ap Gruffudd at the Battle of Mynydd Carn in 1081, the Normans moved into eastern part of Caradog's old dominion and occupied Gwynllwg. Caradog's son Owain Wan (Owain the Weak) retained the upland commote of Machen but the lowlands became a Norman lordship, eventually becoming known as the lordship of Newport after the principal town established by the Norman colonists.
Despite being part of Morgannwg, that is Glamorganshire for centuries, the modern territory of Gwynllwg was parcelled off into Monmouthshire when
Wales was shired and now forms part of the modern county borough of Newport.