Oswiu, also known as Oswy or Oswig (Old English: Ōswīg; c. 612 – 15 February 670), was King of Bernicia from 642 and of Northumbria from 654 until his death. He is notable for his role at the Synod of Whitby in 664, which ultimately brought the church in Northumbria into conformity with the wider Catholic Church.

One of the sons of Æthelfrith of Bernicia and Acha of Deira, Oswiu became king following the death of his brother Oswald in 642. Unlike Oswald, Oswiu struggled to exert authority over Deira, the other constituent kingdom of medieval Northumbria, for much of his reign.

Oswiu and his brothers were raised in exile in the Irish kingdom of Dál Riata in present-day Scotland after their father's death at the hands of Edwin of Northumbria (not by Edwin but possibly by Rædwald and his son Rægenhere at the Battle of the River Idle) only returning after Edwin's death in 633. Oswiu rose to the kingship when his brother Oswald was killed in battle against Penda of Mercia. The early part of his reign was defined by struggles to assert control over Deira and his contentious relationship with Penda, his overlord. In 655, Oswiu's forces killed Penda in a decisive victory at the Battle of the Winwaed, establishing Oswiu as one of the most powerful rulers in Britain. He secured control of Deira, with his son Alhfrith serving as a sub-king, and for three years, Oswiu's power extended over Mercia, earning him recognition as bretwalda over much of Great Britain.

Oswiu was a devoted Christian, promoting the faith among his subjects and establishing a number of monasteries, including Gilling Abbey and Whitby Abbey. He was raised in the Celtic Christian tradition of much of the Irish world, rather than the Roman tradition practiced by the southern Anglo-Saxon kingdoms as well as some members of the Deiran nobility, including Oswiu's queen Eanflæd. In 664, Oswiu presided over the Synod of Whitby, where clerics debated over the two traditions, and helped resolve tension between the parties by decreeing that Northumbria would follow the Roman style. Oswiu died in 670 and was succeeded by his son, Ecgfrith. His feast is 15 February in the East and in the West.

Oswiu was born circa 612, as he was 58 at his death in 670, according to Bede. He was the third child of Æthelfrith, then King of Bernicia; his siblings included older brothers Eanfrith and Oswald and sister Æbbe. Oswiu's mother was likely Æthelfrith's only recorded wife, Acha, a princess of Deira's royal line who is known to have been Oswald's mother. Regardless, his heritage did nothing to endear him to the Deiran nobility; while they accepted Oswald as king apparently on account of his mother, they resisted Oswiu throughout his reign.

At the time of Oswiu's birth, Æthelfrith was at the height of his power. In 604 he had taken control of Deira, evidently by conquest; he killed the previous king (apparently Æthelric), married Acha, a member of the kingly line, and exiled Acha's brother Edwin. His authority ran from the lands of the Picts and the Dál Riata in modern Scotland to Wales and the Midlands in the south. Æthelfrith's power rested on his military success, and this success came to an end in 616, when the exiled Edwin of Northumbria with the support of King Rædwald, defeated and killed him in the battle of the River Idle.

On Æthelfrith's death, his sons and their supporters fled Northumbria, finding sanctuary among the Gaels and Picts of northern Britain and Ireland. Here they would remain until Edwin's death at the Battle of Hatfield Chase in 633.

In exile, the sons of Æthelfrith were converted to Christianity, or raised as Christians. In Oswiu's case, he became an exile at the age of four, and cannot have returned to Northumbria until aged twenty-one, spending childhood and adolescence in a Gaelic milieu. Bede writes that Oswiu was fluent in the Old Irish language and Irish in his faith.