Owain Gwynedd (in English, "Owen") (c. 1100 – November 28, 1170), alternatively known by the patronymic "Owain ap Gruffydd", is considered to be the most successful of all the north Welsh princes prior to his grandson, Llywelyn the Great.

Owain's father, Gruffydd ap Cynan, was a strong and long-lived ruler who had made the principality of Gwynedd the most influential in Wales during the sixty-two years of his reign, using the island of Anglesey as his power base. It was there that Owain was born, about the year 1100. On Gruffydd's death in 1137, therefore, Owain inherited a portion of a well-established kingdom, but had to share it with his brothers. Acting together, they extended the boundaries of their territory. In 1135, they won a major victory over the Normans at Crug Mawr .

After the death of the eldest brother, Cadwallon , in 1132, Owain's main ally was his other brother, Cadwaladr , but the latter's misconduct resulted in his exile, and, from 1143, Owain ruled alone over most of north Wales and a significant part of the south. All went well until the accession of King Henry II of England. Henry had the support of the Pope for his incursions into Wales, partly because Owain had incurred the church's displeasure by taking, as his second wife, his cousin Cristin . He also refused to acknowledge the Archbishop of Canterbury as having jurisdiction over Wales (whose church was much older than that of England). A compromise was eventually achieved, with Owain being officially recognised as ruler of Wales in 1163, in return for paying homage to the English king.

He died in 1170 and was buried in the cathedral at Bangor in north Wales. Following his death, civil war broke out between his sons, and a generation passed before Gwynedd was restored to its former glory. According to legend, one of Owain's sons was Prince Madoc, who is popularly supposed to have fled across the Atlantic and colonised America.