Berkshire ( (listen) BARK-shər, -⁠sheer; in the 17th century sometimes spelt phonetically as Barkeshire; abbreviated Berks.) is a historic county in South East England. One of the home counties, Berkshire was recognised by Queen Elizabeth II as the Royal County of Berkshire in 1957 because of the presence of Windsor Castle, and letters patent were issued in 1974. Berkshire is a county of historic origin, a ceremonial county and a non-metropolitan county without a county council. The county town is Reading.

The River Thames formed the historic northern boundary, from Buscot in the west to Old Windsor in the east. The historic county, therefore, includes territory that is now administered by the Vale of White Horse and parts of South Oxfordshire in Oxfordshire, but excludes Caversham, Slough and five less populous settlements in the east of the Royal Borough of Windsor and Maidenhead. All the changes mentioned, apart from the change to Caversham, took place in 1974. The towns of Abingdon, Didcot, Faringdon, Wallingford and Wantage were transferred to Oxfordshire, the six places joining came from Buckinghamshire. Berkshire County Council was the main local government of most areas from 1889 to 1998 and was based in Reading, the county town which had its own County Borough administration (1888–1974).

Since 1998, Berkshire has been governed by the six unitary authorities of Bracknell Forest, Reading, Slough, West Berkshire, Windsor and Maidenhead and Wokingham. The ceremonial county borders Oxfordshire to the north, Buckinghamshire to the northeast, Greater London to the east, Surrey to the southeast, Wiltshire to the west and Hampshire to the south. No part of the county is more than 8+1⁄2 mi (14 km) from the M4 motorway.