Gruffydd ap Rhys (c. 1081 – 1137) was King of Deheubarth, in Wales. His sister was the Princess Nest ferch Rhys. He was the father of Rhys ap Gruffydd, known as 'The Lord Rhys', who was one of the most successful rulers of Deheubarth during this period.
Gruffydd was born in Llandeilo. Following the death of his father Rhys ap Tewdwr in 1093, Deheubarth was taken over by the Normans, and Gruffydd spent much of his early years in exile in Ireland.
In 1113 Gruffydd visited Gruffudd ap Cynan, King of Gwynedd, at their family seat at Aberffraw on Ynys Môn. The King of Gwynedd owed an honour debt to the House of Dinefŵr, as Gruffydd's father, Rhys ap Tewdwr, had aided Gruffudd ap Cynan in his own bid to reclaim Gwynedd from rivals in 1081. In Aberffraw, Gruffydd ap Tewdwr met Gwenllian, the beautiful youngest daughter of Gwynedd's king. After a brief courtship the couple eloped, with Gwenllian joining Gruffydd ap Rhys on his military campaigns.
After several years, Gruffydd was able to raise enough men to attack a number of Norman castles and towns with some success. However an attack on Aberystwyth was defeated and Gruffydd's army dispersed.
Gruffydd came to terms with King Henry I of England and was allowed to rule a portion of his father's kingdom, the Cantref Mawr, although he was soon under pressure from the Normans again and was forced to flee to Ireland for a period in 1127.