Llywelyn the Great, full name Llywelyn ap Iorwerth, (c. 1172 – 11 April 1240) is a Prince of Gwynedd in north Wales and eventually de facto ruler over most of Wales.

By a combination of war and diplomacy he dominates Wales for 40 years.

During Llywelyn's boyhood, Gwynedd is ruled by two of his uncles, who split the kingdom between them, following the death of Llywelyn's grandfather, Owain Gwynedd, in 1170.

Llywelyn has a strong claim to be the legitimate ruler and begins a campaign to win power at an early age.

He is sole ruler of Gwynedd by 1200 and makes a treaty with King John of England that year.

Llywelyn's relations with John remained good for the next ten years.

He marries John's natural daughter Joan in 1205, and when John arrests Gwenwynwyn ab Owain of Powys in 1208, Llywelyn takes the opportunity to annex southern Powys.

In 1210, relations deteriorate, and John invades Gwynedd in 1211.

Llywelyn is forced to seek terms and to give up all lands east of the River Conwy, but is able to recover them the following year in alliance with the other Welsh princes.

He allies himself with the barons who force John to sign the Magna Carta in 1215.

By 1216, he is the dominant power in Wales, holding a council at Aberdyfi that year to apportion lands to the other princes.

Following King John's death, Llywelyn concludes the Treaty of Worcester with his successor, Henry III, in 1218.

During the next fifteen years, Llywelyn is frequently involved in fights with Marcher lords and sometimes with the king, but also makes alliances with several major powers in the Marches.

The Peace of Middle in 1234 marks the end of Llywelyn's military career, as the agreed truce of two years is extended year by year for the remainder of his reign.

He maintains his position in Wales until his death in 1240 and is succeeded by his son Dafydd ap Llywelyn.