Ælfthryth, also known as Alfreda or Etheldritha,^[1] is a saint, venerated in the Roman Catholic Church, as a virgin, and recluse. She was a daughter of King Offa of Mercia.

According to a late and not very trustworthy legend she was betrothed to Saint Æthelberht, but when he came to the court of Offa to claim her, he was treacherously murdered by the contrivance of Cynethryth, Offa's queen.^[2]

After this Ælfthryth retired to the marshes of Crowland Abbey. She was built into a cell about 793 and lived as a recluse to the end of her days. Crowland claimed her tomb was among those saints arranged around that of Guthlac of Crowland but that her relics were lost in 870 when Danes attacked the monastery. It is impossible not to suspect the existence of some confusion with Ælfleda, another daughter of Offa, whose husband was also murdered by treachery.^[2]