Gruffudd ap Llywelyn (c. 1010 – 5 August 1063) was the only Welsh king to unite all of Wales, ruling from 1055 to 1063. He had previously been King of Gwynedd and Powys from 1039 to 1055. Gruffudd was the son of Llywelyn ap Seisyll, King of Gwynedd, and Angharad, daughter of Maredudd ab Owain, King of Deheubarth, and the great-great-grandson of Hywel Dda. After his death, Wales was again divided into separate kingdoms.

Gruffudd was the son of Llywelyn ap Seisyll, who had been able to rule both Gwynedd and Powys, and of Angharad ferch Maredudd. On Llywelyn's death in 1023, a member of the House of Aberffraw, Iago ab Idwal ap Meurig, became ruler of Gwynedd and began his rise to power in Powys.

In 1039, Iago, King of Gwynedd, was killed (supposedly by his own men), his son Cynan was forced into exile in Dublin, and Gruffudd was made King. Soon after gaining power, he surprised a Mercian army at Rhyd y Groes near Welshpool and defeated it, killing Edwin, brother of Leofric, Earl of Mercia. He then attacked Dyfed, which his father had ruled but which was now under Hywel ab Edwin. Gruffudd again defeated Hywel in the Battle of Pencader in 1041 (halfway between Carmarthen and Lampeter but did not win entirely until 1042 at "Pwlldyfach" (near Carmarthen) and carried off Hywel's wife. Gruffudd seems to have been able to drive Hywel (and his Irish fleet of "Black Gentiles / Pagans") out of the south, for in 1044 Hywel is again recorded returning to the River Towy with a fleet from Ireland; Gruffudd, however, defeated and killed Hywel.

Gruffudd ap Rhydderch of Gwent was able to expel Gruffudd ap Llywelyn from Deheubarth in 1047 and became King of Deheubarth himself. Afterwards the nobles of Ystrad Tywi had attacked and killed 140 of Gruffudd ap Llywelyn's household guards, Gruffudd exacted his revenge in Towy and Dyfed. Gruffudd ap Llywelyn was active on the Welsh border in 1052 when he attacked Herefordshire with an army consisting of a fleet of 18 ships from Ireland; they defeated a mixed force of Normans and Anglo-Saxons in the Battle of Leominster.

According to Brut y Tywysogion, Sweyn Godwinson was called in to help Gruffudd's brother Rhys against Gruffudd ap Rhydderch in 1045 to keep hold of Deheubarth. Gruffudd raided Leominster in 1052, resulting in the Battle of Llanllieni, the Welsh fighting against Normans and Anglo-Saxons.

In 1055 Gruffudd ap Llywelyn killed his rival Gruffudd ap Rhydderch in battle and recaptured Deheubarth. Gruffudd allied himself with Ælfgar, son of Leofric, Earl of Mercia, who had been deprived of his earldom of East Anglia by Harold Godwinson and his brothers. They marched on Hereford and were opposed by a force led by the Earl of Hereford, Ralph the Timid. mounted and armed in the Norman fashion, but on 24 October 1055 Gruffudd and Ælfgar defeated it. They then sacked the city, burned it, and destroyed its motte-and-bailey castle. Earl Harold was given the task of counter-attacking, but Gruffudd and Ælfgar had retreated to South Wales whilst Harold ventured no further than Hereford. He seems here to have built a fortification at Longtown in Herefordshire before refortifying Hereford. Shortly afterwards, Ælfgar was restored to his earldom and a peace treaty was concluded.^{[citation needed]}

Around this time Gruffudd was also able to seize Morgannwg and Gwent, along with extensive territories along the border with England.

The historian John Davies stated that Gruffudd was