Æthelbald (also spelled Ethelbald or Aethelbald; died 757) was the King of Mercia, in what is now the English Midlands from 716 until he was killed in 757. Æthelbald was the son of Alweo and thus a grandson of King Eowa. Æthelbald came to the throne after the death of his cousin, King Ceolred, who had driven him into exile. During his long reign, Mercia became the dominant kingdom of the Anglo-Saxons, and recovered the position of pre-eminence it had enjoyed during the strong reigns of Mercian kings Penda and Wulfhere between about 628 and 675.
When Æthelbald came to the throne, both Wessex and Kent were ruled by stronger kings, but within fifteen years the contemporary chronicler Bede describes Æthelbald as ruling all England south of the Humber estuary. 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.