Sussex, from the Old English SuÞseaxe ('South Saxons'), is an historic county in South East England corresponding roughly in area to the ancient Kingdom of Sussex. Clockwise, it is bounded to the west by Hampshire; north by Surrey, north-east by Kent, south by the English Channel and is divided for local government into West Sussex and East Sussex and the city of Brighton and Hove. Brighton and Hove was created as a unitary authority in 1997, and was granted City status in 2000. Until then, Chichester had been Sussex's only city.

Sussex has three main geographic sub-regions, each orientated approximately east to west. In the south-west of the county lies the fertile and densely populated coastal plain. North of this lie the rolling chalk hills of the South Downs, beyond which lies the well-wooded Sussex Weald.

The name 'Sussex' derives from the Kingdom of Sussex, founded by Ælle of Sussex in 477 AD, which in 825 was absorbed into the kingdom of Wessex and the later kingdom of England. The region's roots go back further to the location of some of Europe's earliest hominid finds at Boxgrove. Sussex has been a key location for England's major invasions, including the Roman invasion of Britain and the Battle of Hastings.

In 1974, the Lord-Lieutenant of Sussex was replaced with one each for East and West Sussex, which became separate ceremonial counties. Sussex continues to be recognised as a geographical territory and cultural region. It has had a single police force since 1968 and its name is in common use in the media. In 2007, Sussex Day was created to celebrate Sussex's rich culture and history. Based on the traditional emblem of Sussex, a blue shield with six gold martlets, the flag of Sussex was recognised by the Flag Institute in 2011. In 2013, Secretary of State for Communities and Local Government Eric Pickles formally recognised and acknowledged the continued existence of England's 39 historic counties, including Sussex.