unitary authority, geographic county of East Sussex, historic county of Sussex, southeastern England. It is located on the English Channel 51 miles (82 km) south of London, with which it is closely linked by rail and superhighway. The unitary authority, which is the largest in population on the south coast of England, comprises the seaside resorts of Brighton and Hove and surrounding communities. It is the main commercial centre of Sussex.

The unitary authority extends from the coast of the English Channel coast into the steep slopes of the South Downs to the north. It adjoins the administrative counties of East Sussex to the east and West Sussex to the west. The University of Sussex is located at the northeastern edge of the unitary authority. The most important sectors of the economy are tourism and financial services. Area 32 square miles (83 square km). Pop. (2005 est.) 255,000.