Rhys ap Gruffydd or ap Gruffudd (often anglicised to "Griffith") (1132 – 28 April 1197) 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.
Rhys's grandfather, Rhys ap Tewdwr, was king of Deheubarth, and was killed at Brecon in 1093 by Bernard de Neufmarché. Following his death, most of Deheubarth was taken over by the Normans. Rhys's father, Gruffydd ap Rhys, eventually was able to become ruler of a small portion, and more territory was won back by Rhys's older brothers after Gruffydd's death. Rhys became ruler of Deheubarth in 1155. He was forced to submit to King Henry II of England in 1158. Henry invaded Deheubarth in 1163, stripped Rhys of all his lands and took him prisoner. A few weeks later he was released and given back a small part of his holdings. Rhys made an alliance with Owain Gwynedd and, after the failure of another invasion of Wales by Henry in 1165, was able to win back most of his lands.
Rhys ap Gruffydd (1508–1531) was a powerful Welsh landowner who was accused of rebelling against King Henry VIII by plotting with James V of Scotland to become Prince of Wales. He was executed as a rebel. He married Lady Catherine Howard (b. abt 1499 Ashwellthorpe, Norfolk, England), the daughter of Thomas Howard, 2nd Duke of Norfolk and his second wife Agnes Tilney.
Rhys was the grandson of Rhys ap Thomas, the most powerful man in Wales and close ally of Henry VII. Rhys was a descendant of the medieval Welsh king Rhys ap Gruffydd (1132–1197), his namesake. His father, Gruffydd ap Rhys ap Thomas, died in 1521, leaving him his grandfather's heir. In 1524 Rhys married Catherine Howard, daughter of Thomas Howard, 2nd Duke of Norfolk.
As his grandfather's heir, Rhys expected to inherit his estates and titles. When Rhys ap Thomas died in 1525, Henry VIII gave his most important titles and powers to Walter Devereux, Lord Ferrers, leading to a feud between Rhys and Ferrers which escalated over the next few years.