Æthelthryth (d. 679) was the daughter of Anna, King of East Anglia, and wife of Ecgfrith, the King of Northumbria.

She took the veil shortly after Ecgfrith's accession, a step which possibly led to his long quarrel with Wilfrid archbishop of York. Ecgfrith married a second wife, Eormenburg, before 678, the year in which he expelled Wilfrid from his kingdom.

According to the Anglo-Saxon Chronicle, she founded the monastery at Ely in 673. Bede tells how after her death, her bones were disinterred by her sister and successor Abbess Seaxburh and buried in a white, marble coffin from Cambridge.