| Consort: | yes |
| Ælfflæd | |
| Succession: | Queen consort of Northumbria |
| Reign: | 792–796 |
| Spouse: | Æthelred I of Northumbria (m. 792) |
| Father: | Offa of Mercia |
| Mother: | Cynethryth |
Ælfflæd was a daughter of Offa of Mercia and Cynethryth.
She may have witnessed a charter with her father, mother, and brother Ecgfrith in the 770s. She certainly witnessed a charter in 787 with her mother, father, brother, and two sisters; here she is described as virgo - unmarried.
It is possible that she was the daughter of Offa whose proposed marriage to Charles the Younger caused a dispute between Charlemagne and Offa in around 789 - 790.
In 792 she married Æthelred I of Northumbria at Catterick. Here she is described as "queen", which has suggested to some historians that she had been previously married, and to a king, perhaps to one of Æthelred's predecessors.