king of Bernicia (from 592/593) and of Deira, which together formed Northumbria.

Aethelfrith was the son of Aethelric and grandson of Ida, king of Bernicia, and his reign marks the true beginning of the continuous history of a united Northumbria and, indeed, of England. He married Acha, daughter of Aelle, king of Deira, whom his father succeeded in 588 or 590, expelling Aelle's son Edwin.

In 604 Aethelfrith repelled the attack of Aidan, king of the Dalriada Scots, at Degsastan, defeating him with great loss. The appearance of Hering, son of Aethelfrith's predecessor, on the side of the invaders seems to indicate family quarrels in the royal house of Bernicia. Later in his reign, probably in 616, he defeated the Welsh in a great battle at Chester and massacred the monks of Bangor who were assembled to aid the Welsh by prayers. This war may have been partly due to Aethelfrith's persecution of Edwin, but it had a strategic importance in the separation of the North Welsh from the Strathclyde Britons. In c. 616 Aethelfrith was defeated and slain at the River Idle by Raedwald, king of East Anglia, whom Edwin had persuaded to take up his cause.