Æthelwulf (Old English for "Noble Wolf"; died 13 January 858) was King of Wessex from 839 to 858. In 825, his father, King Ecgberht, defeated King Beornwulf of Mercia, ending a long Mercian dominance over Anglo-Saxon England south of the Humber. Ecgberht sent Æthelwulf with an army to Kent, where he expelled the Mercian sub-king and was himself appointed sub-king. After 830, Ecgberht maintained good relations with Mercia, and this was continued by Æthelwulf when he became king in 839, the first son to succeed his father as West Saxon king since 641.

The Vikings were not a major threat to Wessex during Æthelwulf's reign. In 843, he was defeated in a battle against the Vikings at Carhampton in Somerset, but he achieved a major victory at the Battle of Aclea in 851. In 853, he joined a successful Mercian expedition to Wales to restore the traditional Mercian hegemony, and in the same year, his daughter Æthelswith married King Burgred of Mercia. In 855, Æthelwulf went on a pilgrimage to Rome. In preparation he gave a "decimation", donating a tenth of his personal property to his subjects; he appointed his eldest surviving son Æthelbald to act as King of Wessex in his absence, and his next son Æthelberht to rule Kent and the south-east. Æthelwulf spent a year in Rome, and on his way back he married Judith, the daughter of the West Frankish king Charles the Bald.

When Æthelwulf returned to England, Æthelbald refused to surrender the West Saxon throne, and Æthelwulf agreed to divide the kingdom, taking the east and leaving the west in Æthelbald's hands. On Æthelwulf's death in 858, he left Wessex to Æthelbald and Kent to Æthelberht, but Æthelbald's death only two years later led to the reunification of the kingdom. In the 20th century, Æthelwulf's reputation among historians was poor: he was seen as excessively pious and impractical, and his pilgrimage was viewed as a desertion of his duties. Historians in the 21st century see him very differently, as a king who consolidated and extended the power of his dynasty, commanded respect on the continent, and dealt more effectively than most of his contemporaries with Viking attacks. He is regarded as one of the most successful West Saxon kings, who laid the foundations for the success of his youngest son, Alfred the Great.