Aldfrith (died December 14, 704) was a King of Northumbria (685 - 704). He was an illegitimate son of Oswiu of Northumbria by Fina, an Irish woman.

When Aldfrith's half-brother, Ecgfrith of Northumbria, died at the Battle of Nechtansmere on May 20, 685, he left behind no heirs. Bede tells a story of Ecgfrith's sister, Elfleda , asking Saint Cuthbert who could possibly take over the kingdom after her brother's death, only to be reminded of the existence of Aldfrith, a scholar who had previously lived a quiet life of study in Ireland and on the island of Iona. Aldfrith subsequently became king.

Although the power of Northumbria had been seriously weakened by the destruction of its army by the Picts at Nechtansmere, Aldfrith is credited with mitigating the damage and stabilizing the kingdom. During his reign, Northumbria enjoyed a degree of peace it had rarely seen in the preceding generations, and under the scholarly Aldfrith (to whom St. Adamnan presented his tract De Locis Sanctis), it seems that literature flourished. His reign was not entirely without war, however; in 698, a Northumbrian ealdorman named Berhtred was killed fighting against the Picts.

The issue of Aldfrith's reign that is best known is his dispute with the bishop Wilfrid. Shortly after becoming king, he invited Wilfrid to return from exile, but a few years later, Wilfrid again went into exile in Mercia. There remained tension between them over the role of the Roman church, of which Wilfrid was a stubborn advocate. The dispute lasted until Aldfrith's death, despite an unsuccessful attempt at reconciliation. He died at Driffield.