Gwynllwg was a kingdom of mediśval Wales and later a Norman lordship and then a cantref. It was named after Gwynllyw, its 6th century ruler and consisted of the coastal plain stretching between the Rhymney and Usk rivers.
The Cathedral at Newport is dedicated to Gwynllyw (corrupted to St. Woolos). The name survives as 'St. Woolos' as the general locality around the Cathedral and as 'Wentloog' in villages such as Peterstone Wentloog and St Brides Wentloog.