|Lord of the Mercians|
|Predecessor||Ceolwulf II (as king)|
|House||House of Mercia|
|Date of death||911 AD|
|Place of burial||St Oswald's Priory, Gloucester|
Æthelred, Lord of the Mercians (or Ealdorman Æthelred of Mercia), (died 911) became ruler of English Mercia shortly after the death of its last king, Ceolwulf II in 879. His rule was confined to the western half, as eastern Mercia was then part of the Viking-ruled Danelaw. Æthelred's ancestry is unknown. He was probably the leader of an unsuccessful Mercian invasion of Wales in 881, and soon afterwards he acknowledged the lordship of King Alfred the Great of Wessex. The alliance was cemented by the marriage of Æthelred to Alfred's daughter Æthelflæd.
In 886 Alfred took possession of London, which had suffered greatly from several Viking occupations; as it had traditionally been a Mercian town, he handed control to Æthelred. In 892 the Vikings renewed their attacks, and the following year Æthelred led an army of Mercians, West Saxons and Welsh to victory over a Viking army at the Battle of Buttington. He spent the next three years fighting them alongside Alfred's son, the future King Edward the Elder. At some time in the decade 899 to 909, Æthelred's health may have declined, and Æthelflæd may have become the effective ruler of Mercia.
After Æthelred's death, Æthelflæd ruled as Lady of the Mercians until her own death in 918. The couple's only child, a daughter called Ælfwynn, then ruled briefly until deposed by her uncle, King Edward.