Æthelred (; died after 704) was king of Mercia from 675 until 704. He was the son of Penda of Mercia and came to the throne in 675, when his brother, Wulfhere of Mercia, died from an illness. Within a year of his accession he invaded Kent, where his armies destroyed the city of Rochester. In 679 he defeated his brother-in-law, Ecgfrith of Northumbria, at the Battle of the Trent: the battle was a major setback for the Northumbrians, and effectively ended their military involvement in English affairs south of the Humber. It also permanently returned the kingdom of Lindsey to Mercia's possession. However, Æthelred was unable to re-establish his predecessors' domination of southern Britain.