Glywysing was, from the sub-Roman period to the Early Middle Ages, a petty kingdom in south-east Wales.

Its people were descended from the Iron Age tribe of the Silures, and frequently in union with Gwent, merging to form Morgannwg.

[1][2] According to 12th-century sources, after the death of Glywys, the kingdom was divided into three cantrefs named for his sons:[3] Penychen, Gwynllwg, and Gorfynydd.

[4] With Gwent increasingly overrun by the Norman conquest of Wales, the last native King of Morgannwyg and Glywysing was Iestyn ap Gwrgan (1081–1090), who was subsequently deposed by Robert Fitzhamon.

Glywysing is ruled by the Kings of Gwent until Rhys ap Ithel Iestyn was the last ruler of an independent Morgannwg, which was thereafter in the possession of the Normans and became the lordship of Glamorgan