Æthelred I (alt. Aethelred, Ethelred; Old English: Æthel-ræd, lit. 'noble counsel'; 845/848 – 871) was King of Wessex from 865 until his death in 871. He was the fourth of five sons of King Æthelwulf of Wessex, four of whom in turn became king. Æthelred succeeded his elder brother Æthelberht and was followed by his youngest brother, Alfred the Great. Æthelred had two sons, Æthelhelm and Æthelwold, who were passed over for the kingship on their father's death because they were still infants. Æthelwold later unsuccessfully disputed the throne with Alfred's son and successor, Edward the Elder.
Æthelred's accession coincided with the arrival of the Viking Great Heathen Army in England.