Mersea Island is an island in Essex, England, in the Blackwater and Colne estuaries to the south-east of Colchester. Its name comes from the Old English word meresig, meaning "island of the pool" and thus is tautological. The island is split into two main areas, West Mersea and East Mersea, and connected to the mainland by the Strood, a causeway that can flood at high tide.

The City of Colchester is a local government district with city status in Essex, England, named after its main settlement, Colchester. The district also includes the towns of West Mersea and Wivenhoe and the surrounding rural areas stretching from Dedham Vale on the Suffolk border in the north to Mersea Island in the Colne Estuary in the south.