Æthelbald (also spelled Ethelbald, or Aethelbald) (died 757) is the King of Mercia, in what is now the English Midlands, from 716 until 757.

During his long reign, Mercia becomes the dominant kingdom of the Anglo-Saxons, and recovers the position of preeminence it had enjoyed during the seventh century under the strong Mercian kings Penda and Wulfhere.

Mercian domination of England continues until the end of the eighth century; Offa, the grandson of Æthelbald's cousin Eanwulf, rules for an additional thirty-nine years, starting shortly after Æthelbald's murder.

Æthelbald comes to the throne on the death of his cousin, King Ceolred.

Both Wessex and Kent are ruled by strong kings at that time, but within fifteen years the contemporary chronicler Bede describes Æthelbald as ruling all England south of the river Humber.

The Anglo-Saxon Chronicle does not list Æthelbald as a bretwalda, or "Ruler of Britain", though this may be due to the West Saxon origin of the Chronicle.

St Boniface writes to Æthelbald in about 745, reproving him for various dissolute and irreligious acts.

The subsequent 747 council of Clovesho, and a charter Æthelbald issued at Gumley in 749—which frees the church from some of its obligations—may have been responses to Boniface's letter.

Æthelbald is killed in 757 by his bodyguards.

He is succeeded briefly by Beornrad, of whom little is known, but within a year Offa seizes the throne.