Rhys ap Gruffydd or ap Gruffudd was the ruler of the kingdom of Deheubarth in south Wales from 1155 to 1197. Today, he is commonly known as The Lord Rhys, in Welsh Yr Arglwydd Rhys, although this title may have not been used in his lifetime. He usually used the title "Proprietary Prince of Deheubarth" or "Prince of South Wales", but two documents have been discovered in which he uses the title "Prince of Wales" or "Prince of the Welsh". Rhys was one of the most successful and powerful Welsh princes, and, after the death of Owain Gwynedd of Gwynedd in 1170, the dominant power in Wales.

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.