The Kingdom of the South Saxons, today referred to as the Kingdom of Sussex (/ˈsʌsɪks/; from Middle English: Suth-sæxe, in turn from Old English: Suth-Seaxe or Sūþseaxna rīce, meaning "(land or people of/Kingdom of) the South Saxons"), was one of the seven traditional kingdoms of the Heptarchy of Anglo-Saxon England. On the south coast of the island of Great Britain, it was originally a sixth-century Saxon colony and later an independent kingdom.

The kingdom remains one of the least known of the Anglo-Saxon polities, with no surviving king-list, several local rulers and less centralisation than other Anglo-Saxon kingdoms. The South Saxons were ruled by the kings of Sussex until the country was annexed by Wessex, probably in 827, in the aftermath of the Battle of Ellendun. In 860 Sussex was ruled by the kings of Wessex, and by 927 all remaining Anglo-Saxon kingdoms were ruled by them as part of the new kingdom of England.

The foundation legend of the kingdom of Sussex is that in 477 Ælle and his three sons arrived in three ships, conquering what is now Sussex. Ælle became overlord, or Bretwalda, over the other Anglo-Saxon kingdoms south of the Humber. Historians are divided over whether or not Ælle really existed; however archaeological evidence supports the view that a short-lived expansion of South Saxon authority as far as the Midlands may have taken place in the 5th century.

For much of the 7th and 8th centuries, Sussex suffered invasion attempts by the kingdom of Wessex to its west. King Æðelwealh formed an alliance with Christian Mercia against Wessex, becoming Sussex's first Christian king. With support from St Wilfrid, Sussex became the last major Anglo-Saxon kingdom to become Christian. South Saxon and Mercian forces took control of what are now east Hampshire and the Isle of Wight. Cædwalla of Wessex killed Æðelwealh and "ravaged Sussex by fierce slaughter and devastation". The South Saxons forced Cædwalla from Sussex and were able to lead a campaign into Kent, replacing its king. At that time Sussex could have re-emerged into a regional power. Shortly afterwards, Cædwalla returned to Sussex, killing its king and putting its people into what Bede called "a worse state of slavery". The South Saxon clergy were put under the control of West Saxon Winchester. Only around 715 was Eadberht of Selsey made the first bishop of the South Saxons, after which further invasion attempts from Wessex ensued.

Following a period of rule by King Offa of Mercia, Sussex regained its independence but was annexed by Wessex around 827 and was fully absorbed into the kingdom of Wessex in 860.

The Kingdom of Sussex had its initial focus in a territory based on the former kingdom and Romano-British civitas of the Regni and its boundaries coincided in general with those of the later county of Sussex. For a brief period in the 7th century, the Kingdom of Sussex controlled the Isle of Wight and the territory of the Meonwara in the Meon Valley in east Hampshire. From the late 8th century, Sussex seems to have absorbed the Kingdom of the Haestingas, after the region was conquered by the Mercian king Offa.

A large part of its territory was covered by the forest that took its name from the fort of Anderitum at modern Pevensey, and known to the Romano-British as the Forest of Andred and to the Saxons as Andredsleah or Andredsweald, known today as the Weald. This forest, according to the Anglo-Saxon Chronicle, was 120 miles (190 km) wide and 30 miles (50 km) deep (although probably closer to 90 miles (140 km) wide). It was the largest remaining area of woodland and heath in the territories that became England and was inhabited by wolves, boars and possibly bears. It was so dense that Domesday Book did not record some of its settlements. The heavily forested Weald made expansion difficult but also provided some protection from invasion by neighbouring kingdoms. While Sussex's isolation from the rest of Anglo-Saxon England has been emphasised, Roman roads must have remained important communication arteries across the forest of the Weald. The Weald was not the only area of Sussex that was forested in Saxon times—for example, at the western end of Sussex is the Manhood Peninsula, which in the modern era is largely deforested, but the name is probably derived from the Old English maene-wudu meaning "men's wood" or "common wood" indicating that it was once woodland.

The coastline would have looked different from today. Much of the alluvium in the river plains had not yet been deposited and the tidal river estuaries extended much further inland. It is estimated that the coastal plain may have been at least one mile broader than it is today. Before people reclaimed the tidal marshes in the 13th century the coastal plain contained extensive areas of sea water in the form of lagoons, salt marsh, wide inlets, islands and peninsulas. To the South Saxons of the 5th and 6th centuries this coastline must have resembled their original homeland between coastal Friesland, Lower Saxony and Schleswig-Holstein.