Ælfthryth (also Alfrida, Elfrida or Elfthryth; died 17 November 999, 1000 or 1001) was Queen of the English from her marriage to King Edgar in 964 or 965 until Edgar's death in 975. She was a leading figure in the regency during the minority of her son King Æthelred the Unready between 978 and 984.
Ælfthryth was the first wife of an English king known to have been crowned and anointed as queen since the mid-ninth century. She had two sons with Edgar, Edmund ætheling (who died young) and Æthelred. Ælfthryth was a powerful political figure and post-Conquest historians accused her of orchestrating the murder of her stepson, King Edward the Martyr, in order to place her son Æthelred on the throne. She appeared as a stereotypical bad queen and evil stepmother in many medieval histories.