Ecgfrith (; c. 645 – 20 May 685) was the King of Deira from 664 until 670, and then King of Northumbria from 670 until his death, succeeding his father Oswiu. He ruled over Northumbria when it was at the height of its power, but his reign ended with a disastrous defeat at the Battle of Nechtansmere in which he lost his life.