Æthelbald (died 860) was King of Wessex from 855 or 858 to 860. He was the second of five sons of King Æthelwulf. In 850, Æthelbald's elder brother Æthelstan defeated the Vikings in the first recorded sea battle in English history, but he is not recorded afterwards and probably died in the early 850s. The next year Æthelwulf and Æthelbald inflicted another defeat on the Vikings at the Battle of Aclea. In 855, Æthelwulf went on pilgrimage to Rome and appointed Æthelbald King of Wessex, while Æthelberht, the next oldest son, became King of Kent, which had been conquered by Wessex thirty years earlier.

On his way back from Rome, Æthelwulf stayed for several months with Charles the Bald, King of the West Franks, whose twelve-year-old daughter Judith he married. When he returned to England in 856, Æthelbald refused to give up the crown. Most historians believe that Æthelbald continued to be king of Wessex while Æthelberht gave up Kent to his father, but some think that Wessex itself was divided, with Æthelbald ruling the west and his father the east, while Æthelberht kept Kent. When Æthelwulf died in 858, Æthelbald continued as (or became again) king of Wessex and his brother resumed (or carried on) his kingship of Kent.

Æthelbald married his stepmother Judith. Asser, the biographer of his youngest brother, Alfred the Great, denounced Æthelbald and Judith's union as being "against God's prohibition and Christian dignity, and also contrary to the practice of all pagans", but the marriage does not seem to have been condemned at the time. Æthelbald and Æthelberht appear to have been on good terms: when Æthelbald died in 860, Æthelberht became king of both Wessex and Kent, and they were never again divided.