Æthelred (died after 704) is King of Mercia from 675 until 704.
He is the son of Penda of Mercia and comes to the throne in 675, when his brother, Wulfhere of Mercia, dies.
Within a year of his accession he invades Kent, where his armies destroy the city of Rochester.
In 679, he defeats his brother-in-law, Ecgfrith of Northumbria, at the Battle of the Trent: the battle is a major setback for the Northumbrians, and effectively endstheir military involvement in English affairs south of the Humber.
It also permanently returns the kingdom of Lindsey to Mercia's possession.
However, Æthelred is unable to reestablish his predecessors' domination of southern Britain.
He is known as a pious and religious king, and he makesmany grants of land to the church.
It is during his reign that Theodore, the Archbishop of Canterbury, reorganizs the church's diocesan structure, creating several new sees in Mercia and Northumbria.
Æthelred befriends Bishop Wilfrid of York when Wilfrid is expelled from his see in Northumbria; Æthelred makes Wilfrid Bishop of the Middle Angles during his exile and supports him at the synod of Austerfield in about 702, when Wilfrid argues his case for the return of the ecclesiastical lands he had been deprived of in Northumbria.
Æthelred's wife, Osthryth, is a daughter of King Oswiu, one of the dominant 7th-century Northumbrian kings.
Osthryth is murdered in unknown circumstances in 697, and in 704 Æthelred abdicates, leaving the throne to Wulfhere's son Coenred.
Æthelred becomes a monk at Bardney, a monastery which he had founded with his wife, and is buried there.
Ceolred, who is Æthelred's son (though apparently not by Osthryth), becomes king after Coenred; it is also possible that Æthelred had another son named Ceolwald who was briefly king before Ceolred.
