Ergyng was a Welsh kingdom of the sub-Roman and early medieval period. It is referred to by the English as Archenfield.
The kingdom lay mostly in what is now western Herefordshire in England, its heartland between the River Monnow and River Wye. However, it also spread into modern Monmouthshire and east of the Wye, where sits the old Roman town of Ariconium (at Weston-under-Penyard) from which its name derives. Presumably this was the first capital.
The area was originally part of the kingdom of Glywysing (modern Glamorgan) and Gwent, but seems to have become independent under King Peibio Clafrog in the mid-6th century. Peibio was the grandfather of Saint Dubricius, the first Bishop of Ergyng and an important figure in the establishment of Christianity in South Wales. Dubricius' cousin, Gwrgan Fawr (the Great) was one of its most important monarchs and may have obtained sway over Glamorgan as far as the River Neath. However, his grandson, Athrwys may have been the last monarch of an independent kingdom, which seems to have fallen back under the control of Gwent after his death around 655.
Ergyng eventually became an administrative cantref and, sometime before the Norman conquest of England, was taken over by the English.