Welsh prince, the most outstanding native ruler to appear in Wales before the region came under English rule in 1283.

Llywelyn was the grandson of Owain Gwynedd (d. 1170), a powerful ruler of Gwynedd in northern Wales. While still a child, Llywelyn was exiled by his uncle, David. He deposed David in 1194 and by 1202 had brought most of northern Wales under his control. In 1205 he married Joan, the illegitimate daughter of England's King John (reigned 1199–1216). Nevertheless, when Llywelyn's attempts to extend his authority into southern Wales threatened English possessions, John invaded Wales (1211) and overran most of Gwynedd. The prince soon won back his lands. He secured his position by allying with John's powerful baronial opponents, and his actions helped the barons influence the king's signing of Magna Carta (1215).

Two years after the accession of King Henry III (reigned 1216–72), the English acknowledged that Llywelyn controlled almost all of Wales, but by 1223 they had forced him to withdraw to the north behind a boundary between Cardigan, Dyfed, and Builth, Powys. Many Welsh princes in the south, however, still accepted his overlordship. In his last years the aged Llywelyn turned his government over to his son David (prince of Gwynedd). When Llywelyn died, a chronicler described him as prince of Wales, which he was in fact, if not in law.