The Borough of Eastleigh is a local government district and borough in Hampshire, England, bordering the unitary authority of Southampton, Test Valley, the City of Winchester and the Borough of Fareham. Eastleigh is separated from the New Forest by Southampton Water. Water bounds much of the borough, with Southampton Water and the River Hamble bordering the east and southwest of the district. The built-up nature of neighbouring Southampton and the urban area around the town of Eastleigh contrast with the rural nature of much of the borough, which lies within the Hampshire Basin.
The original Eastleigh borough was formed in 1936 following the incorporation of the former Eastleigh Urban District Council. The borough as it is today was formed in 1974, when the existing Borough of Eastleigh expanded to include part of the former Winchester Rural District as a result of the Local Government Act 1972. The name of the borough was chosen by the children's author, Charlotte Mary Yonge. There are eight parishes within the borough, but some areas are unparished and controlled directly by the borough council, which has 44 seats. The council's headquarters is in the town of Eastleigh itself.