Glywysing was, from the sub-Roman period through to the Early Middle Ages, a petty kingdom in south-east Wales. Its people were descended from the Iron Age tribe of the Silures.

Location and etymology

Glywysing is said to be named after Glywys, a real or legendary early monarch, whose name may continue that of the Romano-British *Glevenses, the territory and citizens of Glevum, or Gloucester. According to twelfth century sources, after the death of Glywys the kingdom was divided into seven cantrefi named for his sons including Cydweli, Gwyr, Margan, Penychen, Gwynllwg and Gorfynydd, though the kingdom of Glwysing still existed. The borders changed over time, but it is generally thought that its lands originally lay between the Afon Llwyd and the River Towy. At times they expanded eastwards to encompass both Gwent and Ergyng, but some time before the early 8th century, Cydweli and Gwyr (Gower) were lost to Dyfed.
Today the area of Glywysing is known as Glamorgan.

Part of the Kingdom of Morgannwg

In the late 10th century, the kingdom merged with Gwent and changed its name to Morgannwg or Gwlad Morgan in honour of its king, Morgan Hen. Glywysing seems to have been a sub-kingdom or principality of the Kingdom of Morgannwg, along with Gwent.
When Gwent and Glywysing were separated again after the death of Morgan Hen from 974-1055, Glywysing was often referred to as Morgannwg, even though Morgannwg was the formal union between Gwent and Glywysing, which did not reoccur until after Gruffydd ap Llywelyn died in 1063.
Possibly the Kings of Glywysing were also Kings of Morgannwg and the Kings of Gwent were semi-independent subjects of the Kings of Morgannwg.
The last King of Glywysing, Gwent and Morgannwg was Iestyn ap Gwrgan, who was deposed by Robert Fitzhamon.