Rhys ap Tewdwr was a king of Deheubarth in Wales and member of the Dinefwr dynasty, a branch descended from Rhodri the Great. Following the Norman Conquest, he had to pay William the Conqueror to keep his kingdom, which lasted until the end of William's reign.

Deheubarth was a regional name for the realms of south Wales, particularly as opposed to Gwynedd. It is now used as a shorthand for the various realms united under the House of Dinefwr, but that Deheubarth itself was not considered a proper kingdom on the model of Gwynedd, Powys, or Dyfed is shown by its rendering in Latin as dextralis pars or as Britonnes dexterales and not as a named land. In the oldest British writers, Deheubarth was used for all of modern Wales to distinguish it from Hen Ogledd, the northern lands whence Cunedda originated.