For the later earl, see Earl Aethelred of Mercia.

Æthelred (died 716) was King of Mercia from 675 to 704. He was a son of Penda and a brother of Wulfhere, whom he succeeded as king.

In 676 he ravaged Kent with fire and sword, destroying monasteries and churches and taking Rochester. Æthelred married Osthryth, the sister of Ecgfrith, king of Northumbria, but in spite of this connection a quarrel arose between the two kings, presumably over the possession of the province of Lindsey, which Ecgfrith had won back at the close of the reign of Wulfhere.

In a battle on the banks of the Trent in 679, the king of Mercia was victorious and regained the province. Aelfwine, the brother of Ecgfrith, was slain on this occasion, but at the intervention of Theodore, archbishop of Canterbury, Æthelred agreed to pay a weregild for the Northumbrian prince and so prevented further hostilities.

Osthryth was murdered in 697 and Æthelred abdicated in 704, choosing Wulfhere's son Cenred as his successor. He then became abbot of Bardney, and, according to Eddius, recommended Wilfrid to Cenred upon his return from Rome. Æthelred died at Bardney.