Ethelbald (or Ęthelbald) (d. 757) was a King of Mercia (716 - 757) in England and a Bretwalda. He was the son of Alweo and claimed to be a grandson of Eowa, brother of Penda.
Ethelbald spent his early years exiled from Mercia, according to the Life of Guthlac of Crowland, who often provided him and his followers shelter. On Ceolred's death, he returned to become king in 716.
Ethelbald was largely successful in his efforts to impose Mercian authority over southern England, and in doing so his reign foreshadowed that of Offa. During the early part of his reign, the rival kingdom of Wessex remained formidable, but this changed following the abdication of Ine of Wessex in 726. A succession dispute among the West Saxons followed, and the successful claimant to the throne, Aethelheard, seems to have subsequently ruled subject to Mercian authority. This may indicate that his claim to the throne had been supported by Ethelbald. There was nevertheless some cause for conflict between the two kings, however, since in 733 Ethelbald undertook an expedition against Wessex and captured the royal manor of Somerton.
In the north, Ethelbald took advantage of the absence of Eadbert in a campaign against the Picts in 740 to invade Northumbria and burn York. Ethelbald was also at war with the Welsh throughout his reign. In 722, a Mercian defeat at the hands of the Welsh was recorded, probably against Ithel ap Morgan of Glywysing. In 743, Ethelbald and Aethelheard's successor in Wessex, Cuthred, campaigned against the Welsh together. Cuthred was determined to overthrow Mercia's domination of Wessex, however, and in 752 he defeated Ethelbald at Beorhford. This gained Wessex a few years of independence, but Ethelbald seems to have reasserted his authority over the West Saxons by the time of his death, since a later West Saxon king, Cynewulf, is recorded as witnessing a charter of Ethelbald at the very beginning of his reign, in 757.
We may gather something of Ethelbald's character and policies from a letter sent to him by Saint Boniface, bishop of Mainz, in either 746 or 747. In it, Boniface reproached Ethelbald for his behavior towards church property, towards nuns, and concerning the example he set for his subjects. It was perhaps in response to these admonitions that, in a charter of 749, Ethelbald freed ecclesiastical lands from all obligations except the trinoda necessitas.
In 757, he was slain by his bodyguard at Seckington, Warwickshire, near the royal seat of Tamworth. His death may have been due to a feud, or it may have been the work of his short-lived successor, Beornrad. Ethelbald was buried at Repton.