The Kingdom of the East Saxons (Old English: Ēastseaxna rīce; Latin: Regnum Orientalium Saxonum), referred to as the Kingdom of Essex /ˈɛsɪks/, was one of the seven traditional kingdoms of the Anglo-Saxon Heptarchy. It was founded in the 6th century and covered the territory later occupied by the counties of Essex, Middlesex, much of Hertfordshire and (for a short while) west Kent. The last king of Essex was Sigered of Essex, who in 825 ceded the kingdom to Ecgberht, King of Wessex.

The Kingdom of Essex was bounded to the north by the River Stour and the Kingdom of East Anglia, to the south by the River Thames and Kent, to the east lay the North Sea and to the west Mercia. The territory included the remains of two provincial Roman capitals, Colchester and London.

The kingdom included the Middle Saxon Province, which included the area of the later County of Middlesex and most, if not all, of Hertfordshire Although the province is ever recorded only as part of the East Saxon Kingdom, charter evidence shows that it was not part of its core territory. In the core area, it granted charters freely, but further west, it did so while making reference to its Mercian overlords. At times, Essex was ruled jointly by co-Kings, and it thought that the Middle Saxon Province is likely to have been the domain of one of the co-kings. The links to Essex between Middlesex and parts of Hertfordshire were long reflected in the Diocese of London, re-established in 604 as the East Saxon see, and its boundaries continued to be based on the Kingdom of Essex until the nineteenth century.

The East Saxons also had intermittent control of Surrey. For a brief period in the 8th century, the Kingdom of Essex controlled west Kent.

The modern English county of Essex maintains the historic northern and the southern borders but covers only the territory east of the River Lea, the other parts being lost to neighbouring Mercia during the 8th century.

In the Tribal Hidage it is listed as containing 7,000 hides.

Although the kingdom of Essex was one of the kingdoms of the Heptarchy, its history is not well documented. It produced relatively few Anglo-Saxon charters and no version of the Anglo-Saxon Chronicle; in fact, the only mention in the chronicle concerns Bishop Mellitus. As a result, the kingdom is regarded as comparatively obscure. For most of the kingdom's existence, the Essex king was subservient to an overlord – variously the kings of Kent, East Anglia or Mercia.

Saxon occupation of land that was to form the kingdom had begun by the early 5th century at Mucking and other locations. A large proportion of the original settlers came from Old Saxony. According to British legend (see Historia Brittonum) the territory known later as Essex was ceded by the Celtic Britons to the Saxons following the infamous Treason of the Long Knives, which occurred c. 460 during the reign of High King Vortigern. Della Hooke relates the territory ruled by the kings of Essex to the pre-Roman territory of the Trinovantes. There is an academic debate about the traditional narrative, with some scholars suggesting a pattern of typically peaceful co-existence, with the structure of the Romano-British landscape being maintained, and with the Saxon settlers believed to have been in the minority. However Alexander Mirrington argues that the cultural change seen in the archaeological record is so complete that "a migration of a large number of people is the most logical and least extreme solution".