Llywelyn the Great (Welsh Llywelyn Fawr) was a Prince of Gwynedd in North Wales and eventually de facto ruler over most of Wales. By a combination of war and diplomacy he dominated Wales for forty years, and was one of only two Welsh rulers to be called 'the Great'.

During Llywelyn's boyhood Gwynedd was ruled by two of his uncles, who had agreed to split the kingdom between them following the death of Llywelyn's grandfather, Owain Gwynedd, in 1170. Llywelyn had a strong claim to be the legitimate ruler and began a campaign to win power at an early age. He was sole ruler of Gwynedd by 1200, and made a treaty with King John of England the same year. Llywelyn's relations with John remained good for the next ten years. He married John's illegitimate daughter Joan in 1205, and when John arrested Gwenwynwyn ab Owain of Powys in 1208 Llywelyn took the opportunity to annex southern Powys. In 1210 relations deteriorated and John invaded Gwynedd in 1211. Llywelyn was forced to seek terms and to give up all his lands east of the River Conwy, but was able to recover these lands the following year in alliance with the other Welsh princes. He allied himself with the barons who forced John to sign Magna Carta in 1215. By 1216 he was the dominant power in Wales, holding a council at Aberdyfi that year to apportion lands to the other princes.