Æthelstan (died c. 852), the eldest son of King Æthelwulf of Wessex, was the King of Kent from 839 under the authority of his father. The late D, E and F versions of the Anglo-Saxon Chronicle describe Æthelstan as Æthelwulf's brother, but the A, B and C versions, and Æthelweard's Chronicon, state that he was Æthelwulf's son. Some historians have argued that it is more probable that he was a brother, including Eric John in 1966 and Ann Williams in 1978. However, in 1991 Ann Williams described him as Æthelwulf's son, and this is now generally accepted by historians, including Frank Stenton, Barbara Yorke, and D. P. Kirby.
When Æthelwulf became King of the West Saxons in 839 on the death of his father, Egbert, he appointed Æthelstan to rule over Kent, Essex, Surrey and Sussex. He is styled king in the Anglo-Saxon Chronicle and Æthelweard's chronicle calls him "King of the Dwellers in Kent, of the East Saxons, of the South Saxons and of Surrey". He attested a number of his father's charters as king in the 840s.
In 851, Æthelstan and Ealdorman Ealhhere defeated a Viking fleet and army at Sandwich, Kent, described by Frank Stenton as "the first naval battle in recorded English history". Ealhhere's death in battle against Vikings is recorded c. 853. Æthelstan is not mentioned after 851 and presumably died before Æthelwulf went to Rome in 855 as he was not included in arrangements for government of the kingdom during his father's absence.