The Borough of Eastleigh is a local government district with borough status in Hampshire, England. It is named after its main town of Eastleigh, where the council is based. The borough also contains the town of Hedge End along with several villages, many of which form part of the South Hampshire urban area.

The neighbouring districts are Fareham, Winchester, Test Valley, Southampton and (across Southampton Water) New Forest. Water bounds much of the borough, with Southampton Water and the River Hamble bordering the east and southwest of the district. The borough 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 borough's Latin motto, "Salus populi suprema lex" translates as "The Welfare of the People is the most important Law".

The borough is served by two motorways and seven railway stations as well as an international airport. There is also a ferry linking Hamble-le-Rice in Eastleigh to Warsash in Fareham, and a disused canal running through the north of the borough.

There are eight scheduled monuments and around 180 listed buildings in the borough, with Netley Abbey, Bursledon Windmill, the chapel of Netley Hospital, and Netley Castle among them. The borough also contains eight conservation areas and around 20,000 trees protected by tree preservation orders.

The borough's origins begin with the formation of an ecclesiastical parish covering the hamlets of Eastley and Barton in 1868, in an area which was beginning to be developed around Bishopstoke railway station which had opened in 1839 to serve the older village of Bishopstoke a mile to the east. Author Charlotte Mary Yonge, a resident of Otterbourne, donated £500 (£60,000 at 2023 values) towards the cost of building a parish church and in return was asked which of the two villages to name the parish after; she chose Eastley, but also chose to alter the spelling to Eastleigh as she considered this more modern. The parish grew rapidly: it had a population of 515 in 1871, over 1,000 in 1881 and 3,613 in 1891.

Despite the creation of the ecclesiastical parish, Eastleigh remained part of the civil parish of South Stoneham. In order to facilitate the creation of pavements with kerbs, drains and sewers, and street lights, a local government district covering the ecclesiastical parish of Eastleigh was established in 1893, governed by a local board. Such districts were converted into urban districts in 1894, when Eastleigh was also made its own civil parish. Eastleigh Urban District was enlarged to take in the community of Bishopstoke in 1899, becoming the Eastleigh and Bishopstoke Urban District. The name was changed back to just Eastleigh in 1932.

The first Eastleigh Borough was incorporated in 1936 under the Municipal Corporations Act 1882. This conversion from Eastleigh Urban District Council to Eastleigh Borough Council allowed the authority to create byelaws and appoint a mayor. Notice of the petition for incorporation was served on the 1 February 1936, and the matter being raised for consideration on 16 March, along with petitions for the creation of boroughs for Crosby and Sale in North West England, among others.