The Kingdom of the West Saxons, also known as the Kingdom of Wessex, was an Anglo-Saxon kingdom in the south of Great Britain, from around 519 until Alfred the Great declared himself as King of the Anglo-Saxons in 886.

The Anglo-Saxons believed that Wessex was founded by Cerdic and Cynric of the Gewisse, though this is considered by some to be a legend. The two main sources for the history of Wessex are the West Saxon Genealogical Regnal List and the Anglo-Saxon Chronicle (the latter of which drew on and adapted an early version of the List), which sometimes conflict. Wessex became a Christian kingdom after Cenwalh (r. 642–645, 648–672) was baptised and was expanded under his rule. Cædwalla later conquered Sussex, Kent and the Isle of Wight. His successor, Ine (r. 689–726), issued one of the oldest surviving English law codes and established a second West Saxon bishopric. The throne subsequently passed to a series of kings with unknown genealogies.

During the 8th century, as the hegemony of Mercia grew, Wessex largely retained its independence. It was during this period that the system of shires was established. Under Egbert, Surrey, Sussex, Kent, Essex, and Mercia, along with parts of Dumnonia, were conquered. He also obtained the overlordship of the Northumbrian king. However, Mercian independence was restored in 830. During the reign of his successor, Æthelwulf, a Danish army arrived in the Thames estuary, but was decisively defeated. When Æthelwulf's son, Æthelbald, usurped the throne, the kingdom was divided to avoid war. Æthelwulf was succeeded in turn by his four sons, the youngest being Alfred the Great.

Wessex was invaded by the Danes in 871, and Alfred was compelled to pay them to leave. They returned in 876, but were forced to withdraw. In 878, they forced Alfred to flee to the Somerset Levels, but were eventually defeated at the Battle of Edington. During his reign Alfred issued a new law code, gathered scholars to his court and was able to devote funds to building ships, organising an army and establishing a system of burhs. Alfred's son, Edward, captured the eastern Midlands and East Anglia from the Danes and became ruler of Mercia in 918 upon the death of his sister, Æthelflæd. Edward's son, Æthelstan, conquered Northumbria in 927, and England became a unified kingdom for the first time. Cnut the Great, who conquered England in 1016, created the wealthy and powerful earldom of Wessex, but in 1066 Harold Godwinson reunited the earldom with the crown and Wessex ceased to exist.

From the Neolithic onwards the chalk downland of the area that would become Wessex was traversed by the Harrow Way, which can still be traced from Marazion in Cornwall to the coast of the English Channel near Dover, and was probably connected with the ancient tin trade. In the Late Neolithic, the ceremonial sites of Avebury and Stonehenge were completed on Salisbury Plain, but the final phase of Stonehenge was erected by the so-called "Wessex culture" of the Middle Bronze Age (c. 1600–1200 BC). The area has many other earthworks and erected stone monuments from the Neolithic and Early Bronze periods, including the Dorset Cursus, an earthwork 10 km (6 mi) long and 100 m (110 yd) wide, which was oriented to the midwinter sunset. Although agriculture and hunting were pursued during this long period, there is little archaeological evidence of human settlements. By the Iron Age, Celtic British tribes such as the Durotriges, Atrebates, Belgae and Dobunni occupied the future Wessex.

Following the Roman conquest of Britain, from the 1st century AD, numerous country Roman villas with attached farms were established across Wessex, along with the important towns of Dorchester and Winchester (the ending -chester comes from Latin castra, "a military camp"). The Romans, or rather the Romano-British, built another major road that integrated Wessex, running eastwards from Exeter through Dorchester to Winchester and Silchester and on to London.

In the mid-4th century there were increasing raids on Roman Britain by peoples such as the Picts, Scottish people, Attacotti, and Franks, as well as the Saxons. In 367, these tribes simultaneously invaded Britain from the north, west and east. The invaders reportedly defeated or co-opted Roman forces in most parts of northern and western Britain. However, the Roman general Theodosius had recaptured most areas by the end of 368. In 380–1, Magnus Maximus defeated further raids. However, there was increasing internal conflict across the Roman Empire. During 383–4, in the context of the overthrow of Emperor Gratian, Maximus took most of the garrison from Britain to Gaul, where he was made Augustus of the West, ruling Britain, Gaul, Spain and Roman Africa. Following the death of Maximus in 388, Roman authority in Britain again declined. During the late 390s, Stilicho attempted to restore control, with a campaign against the Picti, but this was undermined in 401 when Stilicho transferred troops to the Continent to fight the Goths. Two subsequent Roman rulers of Britain were murdered. In 407, a Roman officer in Britain, Constantine III declared himself Augustus of the West, and left for Gaul, taking with him Roman troops. Finally, in 410, when Romano-British officials requested military assistance from Emperor Honorius, he told them to manage their own defences. Economic decline occurred after these events: circulation of Roman coins ended and the importation of items from the Roman Empire stopped.

Theories about the settlement of Saxons, Jutes and Angles in Britain are divided into two categories by the historian Peter Hunter Blair (1956), namely "Welsh" and "English".