Oswiu (c. 612–February 15, 670; also rendered Oswio, Oswy, and Osuiu) was an Anglo-Saxon Bretwalda. He reigned as king of Northumbria from 642 to 670 and was the seventh of the great English kings enumerated by Bede.

Oswiu was the son of Æthelfrith and brother of Oswald, whom he succeeded in Bernicia in 642 after the Battle of Maserfeld.

Oswiu succeeded in subjugating many of the Britons, Picts and Scots. At Gilling in 651, he plotted the murder of Oswine, a relative of Edwin who had become king of Deira, and a few years later took possession of that kingdom. He appears to have consolidated his power by the aid of the Roman Church Church and by a series of judicious matrimonial alliances. It was probably in 642 that he married Eanfled , daughter of Edwin, thus uniting the two rival dynasties of Northumbria. His daughter Alhfled he married to Peada, son of Penda, the king of Mercia; another daughter, Osthryth, became the wife of Æthelred, Penda's third son. Oswiu was chiefly responsible for the reconversion of the East Saxons. He is said to have convinced their their king, Sigeberht II , of the truth of Christianity by his arguments, and at his request sent Cedd , a brother of Ceadda (St. Chad), on a mission to Essex.

In 655, Oswiu was attacked by Penda, and, after an unsuccessful attempt to buy him off, defeated and slew the Mercian king at the Battle of Winwaed. He then took possession of the northern part of Mercia, giving the southern portion to Penda's son Peada. As a thanksgiving offering he dedicated his daughter Rifled to the Church, and founded the monastery of Whitby. About this time he is thought by many to have obtained some footing in the kingdom of the Picts through their king Talorcan, the son of his brother Eanfrith. In 660 he married his son Ecgfrith to Æthelthryth, daughter of the former East Anglian king Anna. In 664 at the synod of Whitby, Oswiu accepted the usages of the Roman Church, which led to the departure of Colman and the appointment of Wilfrid as bishop of York. Oswiu died in 670 and was succeeded by his son Ecgfrith.